All of us make mistakes. As Isaiah in the Bible says, "All we like sheep have gone astray" (Isaiah 53:6). It also says in 1 John 1:8, "If we say that we have no sin, we deceive ourselves, and the truth is not in us."
If we repent from our mistakes and try our best never to do them again, God will forgive us, and His judgement will be merciful. God said, "Behold, he who has repented of his sins, the same is forgiven, and I, the Lord, remember them no more. By this ye may know if a man repenteth of his sins-behold, he will confess them and forsake them." (Doctrine and Covenants 58:42-43). ("Forsake" means to leave altogether or to abandon.) We can repent because of Christ's sacrifice for us. To repent, we first have to have faith in Christ.
If we don't repent, God's judgement will be harsh and we won't be given mercy. In the Bible it states, "And shall come forth; they that have done good, unto the resurrection of life; and they that have done evil, unto the resurrection of damnation" (John 5:29). The resurrection of damnation means that we will be stopped in our eternal progress (just like a dam stops the water in a reservoir). If we don't repent and change our lives for good, God's harsh judgement will be on us.
Learn more about John by going to our Bible quiz page.
In the Book of Mormon (Alma 34:34) it says, "Ye cannot say, when ye are brought to that awful crisis, that I will repent, that I will return to my God. Nay, ye cannot say this; for that same spirit which doth possess your bodies at the time that ye go out of this life, that same spirit will have power to possess your body in that eternal world."
Remember, to repent, it means that you stop committing the sin. Learn more about the steps to repentance