Turning from what they know
We are accountable for acting in complete alignment with our most sincere idea of God's character as applied to our lives. Jesus atoned for the sins you do not know of, and while he provides the ability for you to be forgiven for every sin you forsake, he will not save you while you willfully rebel against him.
Dwindle in unbelief
You dwindle in unbelief when you reject something the Lord tells you. Rejecting just one thing the Lord tells you brings you out of alignment with God until or unless you repent. Furthermore, it makes it more likely for you to do something worse, because you are dimming the light inside you making it harder for you to see the right course of action in any given situation. Thus, turning away tends to be cyclical and not a once off.
King David provides an unfortunate example of what happens when we turn from what we know. Upon learning she was married, David should have left Bathsheba alone. But he didn't. He turned away from what his conscience told him. Doing so opened the door to greater sin, because it dimmed the light within him, making it possible for him to do more than he otherwise would have. David never would have considered arranging for Uriah's death if he hadn't first turned away from the spirit in seeking Bathsheba after learning she was married.
All of us have access to the spirit of the Lord, which tells us the difference between right and wrong through our conscience. When we offend this often subtle source, we must rely on signs in this world to help us see where we have been foolish. Typically, these include awful afflictions and trials sent to us from a God that loves us enough to expose us to the consequences of our rebellious and foolish actions in hopes that we will repent. Most times, the rebellious will be blind to these afflictions, blaming them on others or pretending that they are just unlucky. This is called "kicking against the pricks."
Sometimes, he sends human messengers to us in obvious and non-obvious ways to snap us out of our rebellion. While the spirit speaks in a still, small, easily-offended voice, God's servants have "faces like flint." They do not care who they offend. They pay the price, having exercised exceeding mercy beyond what the spirit can in obtaining and delivering God's word in plainness and boldness, in hopes that their intercession--which almost always results in rejection and persecution--will help the person who is persisting in rebellion.
Outward vs. inward repentance
Sorrow does not repentance make. Laman and Lemuel sorrowed deeply for their sins when severely chastised on multiple occasions. Did they actually change? They did not.
People can choose how they react to an invitation to repent. Most ignore it completely. Some react with sorrow, but not because they love God more than they did previously. Rather, their reaction is driven by their desire to avoid the consequence of sin. They want to feel good, not be good.
This type of surface sorrowing is described in the scriptures as the "sorrowing of the damned":
10 And it came to pass that the Nephites began to repent of their iniquity, and began to cry even as had been prophesied by Samuel the prophet; for behold no man could keep that which was his own, for the thieves, and the robbers, and the murderers, and the magic art, and the witchcraft which was in the land.
11 Thus there began to be a mourning and a lamentation in all the land because of these things, and more especially among the people of Nephi.
12 And it came to pass that when I, Mormon, saw their lamentation and their mourning and their sorrow before the Lord, my heart did begin to rejoice within me, knowing the mercies and the long‐suffering of the Lord, therefore supposing that he would be merciful unto them that they would again become a righteous people.
13 But behold this my joy was vain, for their sorrowing was not unto repentance, because of the goodness of God; but it was rather the sorrowing of the damned, because the Lord would not always suffer them to take happiness in sin.
14 And they did not come unto Jesus with broken hearts and contrite spirits, but they did curse God, and wish to die. Nevertheless they would struggle with the sword for their lives. (Mormon 2)
Those who experience the sorrow of the damned may go through great lengths to feign repentance. When King Saul sinned in disobeying God's direct command to kill all the living things in the kingdom of the Amalekites, Saul tried to defend his disobedience by saying he had spared the best to give a sacrifice to the Lord:
17 And Samuel said, When thou wast little in thine own sight, wast thou not made the head of the tribes of Israel, and the Lord anointed thee king over Israel?
18 And the Lord sent thee on a journey, and said, Go and utterly destroy the sinners the Amalekites, and fight against them until they be consumed.
19 Wherefore then didst thou not obey the voice of the Lord, but didst fly upon the spoil, and didst evil in the sight of the Lord?
20 And Saul said unto Samuel, Yea, I have obeyed the voice of the Lord, and have gone the way which the Lord sent me, and have brought Agag the king of Amalek, and have utterly destroyed the Amalekites.
21 But the people took of the spoil, sheep and oxen, the chief of the things which should have been utterly destroyed, to sacrifice unto the Lord thy God in Gilgal.
22 And Samuel said, Hath the Lord as great delight in burnt offerings and sacrifices, as in obeying the voice of the Lord? Behold, to obey is better than sacrifice, and to hearken than the fat of rams.
23 For rebellion is as the sin of witchcraft, and stubbornness is as iniquity and idolatry. Because thou hast rejected the word of the Lord, he hath also rejected thee from being king.
24 And Saul said unto Samuel, I have sinned: for I have transgressed the commandment of the Lord, and thy words: because I feared the people, and obeyed their voice.
25 Now therefore, I pray thee, pardon my sin, and turn again with me, that I may worship the Lord.
26 And Samuel said unto Saul, I will not return with thee: for thou hast rejected the word of the Lord, and the Lord hath rejected thee from being king over Israel. (1 Samuel 15)
A person's sacrifice does not matter. It doesn't matter if someone says they are sorry or weeps or does extreme things in response to their realization that they have sinned. What matters is if they are now willing to obey God. This is the only sacrifice that God is interested in: the heart of the person.
You can tell when someone is sorrowing but not unto repentance when they do not forsake their sins. They'd rather die than do it again. For example, the converted Lamanites who buried their weapons of war (see Alma 24).
Real repentance is a matter of the heart. When a person really repents, they change their core. The truly repentant address the core issue. If their sin is caused by selfishness, for example, they stop being selfish. They fix the problem, not the symptoms of it.
This manifests in all the outward behaviors that are in any way relevant to the core problem. When someone repents only at the surface level, they may chance a specific behavior or two, but you will see that their hearts are not changed because the same problem will continue to manifest in other behaviors.
If someone's repentant behavior changes back once the external stimulus is removed, then they have not truly repented. If they repeat their sin in any way, they have not truly repented.
Exercise extreme care with those who only surface-repent. Their hearts are not only still capable of the sins they previously committed, but they are now capable of worse sin. They have not only lost even more light through their willful disobedience, but they are under the false impression that they are better than they were previously while actually being worse. These people can continue flailing in the dark towards offenses that were previously unthinkable for them to commit.
What happens if they don't repent?
There comes a point where God stops trying to help these people and turns them over to the full weight of the consequences of their choices, a situation called "the buffetings of Satan" in the scriptures. When does this happen? When mercy ceases to benefit. This happens when, for example:
1. The person's prolonged behavior will break God's promises. For instance, the Lamanites who tried to kill Ammon were instead killed because God had promised to preserve his life.
2. When justice has built up to a point where it absolutely must be paid out if it is not be broken, such as when the continuous righteous obedience of the oppressed reaches critical mass. This happened, for example, when Abigails righteousness so exceeded Nabal's that God killed Nabal to both free her from his oppression and to clear the path for her to have a righteous husband (David).
3. When the continuous and deepening wickedness of the person has caused them to remove themselves from the channels God could use to reach them, such as when the wicked cast out the righteous from their cities, cutting themselves off from the only source they have to hear God's correcting voice, having long ago offended his spirit to the point of being past feeling it.
Sometimes, the consequences of their choices include an ignominious death, as when Saul was killed in battle or Korihor was trampled.
There is a point where it is too late
Many blessings have an appointed time and place. Sometimes, it is too late to go back to what you could have had.
The example of Saul is illustrative of time-constrained consequences. Saul was anointed king over Israel, but because of his disobedience and rebellion, he was deposed and replaced.
1 Saul reigned one year; and when he had reigned two years over Israel,
2 Saul chose him three thousand men of Israel...
3 ...And Saul blew the trumpet throughout all the land, saying, Let the Hebrews hear.
4 And all Israel heard say that Saul had smitten a garrison of the Philistines, and that Israel also was had in abomination with the Philistines. And the people were called together after Saul to Gilgal.
5 And the Philistines gathered themselves together to fight with Israel, thirty thousand chariots, and six thousand horsemen, and people as the sand which is on the sea shore in multitude....
6 When the men of Israel saw that they were in a strait, (for the people were distressed,) then the people did hide themselves in caves, and in thickets, and in rocks, and in high places, and in pits.
7 And some of the Hebrews went over Jordan to the land of Gad and Gilead. As for Saul, he was yet in Gilgal, and all the people followed him trembling.
8 And he tarried seven days, according to the set time that Samuel had appointed: but Samuel came not to Gilgal; and the people were scattered from him.
9 And Saul said, Bring hither a burnt offering to me, and peace offerings. And he offered the burnt offering.
10 And it came to pass, that as soon as he had made an end of offering the burnt offering, behold, Samuel came; and Saul went out to meet him, that he might salute him.
11 And Samuel said, What hast thou done? And Saul said, Because I saw that the people were scattered from me, and that thou camest not within the days appointed, and that the Philistines gathered themselves together at Michmash;
12 Therefore said I, The Philistines will come down now upon me to Gilgal, and I have not made supplication unto the Lord: I forced myself therefore, and offered a burnt offering.
13 And Samuel said to Saul, Thou hast done foolishly: thou hast not kept the commandment of the Lord thy God, which he commanded thee: for now would the Lord have established thy kingdom upon Israel for ever.
14 But now thy kingdom shall not continue: the Lord hath sought him a man after his own heart, and the Lord hath commanded him to be captain over his people, because thou hast not kept that which the Lord commanded thee. (1 Samuel 13)
Similarly, Saul sought the life of David, who righteously fled from him and would not fight back, and in doing so incurred the penalty of his own death. Saul repeatedly tried to kill David, who was not only innocent, but was also the Lord's anointed, and also repeatedly forgave Saul by not killing him when he had the chance and would have been justified. Ironically, David did not kill Saul because Saul was the Lord's anointed. All of these factors worked together to cause an irreversible decree from heaven: Saul would surely die due to his crimes and David's consistent righteousness. The Lord's justice could no longer be withheld.
No matter how blind you choose to be, there will come a day when you are shown in undeniable plainness how often God reached out for you, and you would not listen. You will see the price that has been paid for you by the Lord Jesus so that you could have a way out of your sins. You will see the price paid by his servants when they tried again and again to put the weight of your sin on their own backs to give you yet another chance to repent. You will be shown in undeniable plainness the blessings that were prepared for and offered you, but you would not. In that day, you will find yourself in a hell you have prepared for yourself, an awful jail cell of your own creation.
I implore you to inspect your life for willful disobedience and to purge it from yourself. Do not allow yourself to believe your own lies that your surface repentance is sufficient. Come to Jesus Christ will all your heart, might, mind, and strength, and be truly healed of your weakness through the things he will teach you. Expose all of your darkness to his light, and rejoice in the cleansing burning it brings.