Short of totally rejecting God and His way of life, if we desire to change and repent, all sins can be forgiven. One of the most wonderful truths about the grace of God is that whatever we might have done in life can be forgiven.
We can know that when we repent, our sin is washed away to be forgiven and remembered against us no longer. As Hebrews 10:19-22 Hebrews 10:19-22  Having therefore, brothers, boldness to enter into the holiest by the blood of Jesus,
 By a new and living way, which he has consecrated for us, through the veil, that is to say, his flesh;
 And having an high priest over the house of God;
 Let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience, and our bodies washed with pure water.
American King James Version×tells us: “Therefore brethren, having boldness to enter the Holiest [coming before God’s heavenly throne in our prayers] by the blood of Jesus, by a new and living way which He consecrated for us . . . and having a High Priest over the house of God, let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience and our bodies washed with pure water.”
We see here that faith in the sacrifice of Jesus Christ lets us have full assurance and faith that by it we can be washed clean from our sin (as now symbolized by baptism), to have all guilt removed, and to have a pure, clean and clear conscience as we come before God.
Once our old self is symbolically buried through baptism, God wants us to put behind us feelings of guilt over past sins. He wants us to approach our future with confidence that our sins have been forgiven. We are to begin a new life without worry and guilt over the past.
God offers us freedom from the burden of uncertainty so we can know for sure where we stand with Him. Once we have accepted Jesus Christ and been forgiven of our sins (and repent of any further sins), our conscience should be clear and washed clean.
Prolonged and obsessive guilt is often a major but preventable cause of emotional agony. Some guilt is natural and good if it leads to repentance. If we feel bad for something we did, godly sorrow that leads to repentance is a good kind of sorrow (see 2 Corinthians 7:9-11 2 Corinthians 7:9-11  Now I rejoice, not that you were made sorry, but that you sorrowed to repentance: for you were made sorry after a godly manner, that you might receive damage by us in nothing.
 For godly sorrow works repentance to salvation not to be repented of: but the sorrow of the world works death.
 For behold this selfsame thing, that you sorrowed after a godly sort, what carefulness it worked in you, yes, what clearing of yourselves, yes, what indignation, yes, what fear, yes, what vehement desire, yes, what zeal, yes, what revenge! In all things you have approved yourselves to be clear in this matter.
American King James Version×).
The solution to guilt over sin is to go to God in a repentant attitude for the grace and forgiveness He promises, do what we can to make amends with others we may have hurt in some way and focus on being a new person going forward—letting go of who we were before and ceasing to dwell on past mistakes.
As Proverbs 24:16 Proverbs 24:16For a just man falls seven times, and rises up again: but the wicked shall fall into mischief.
American King James Version×tells us, “A righteous man may fall seven times and rise again.” We just need to get back up and keep going, seeking God’s mercy regularly and frequently.
As God promised us in Isaiah 1:18 Isaiah 1:18Come now, and let us reason together, said the LORD: though your sins be as scarlet, they shall be as white as snow; though they be red like crimson, they shall be as wool.
American King James Version×, “Though your sins are like scarlet, they shall be as white as snow; though they are red like crimson, they shall be [white] as wool.”
Psalms 103:12 Psalms 103:12As far as the east is from the west, so far has he removed our transgressions from us.
American King James Version×tells us, “As far as the east is from the west, so far has He removed our transgressions from us.”
In Philippians 3:13-15 Philippians 3:13-15  Brothers, I count not myself to have apprehended: but this one thing I do, forgetting those things which are behind, and reaching forth to those things which are before,
 I press toward the mark for the prize of the high calling of God in Christ Jesus.
 Let us therefore, as many as be perfect, be thus minded: and if in any thing you be otherwise minded, God shall reveal even this to you.
American King James Version×Paul describes the attitude of a clear conscience that God wants for us: “Brethren, I do not count myself to have apprehended [or fully achieved what God intends for us]; but one thing I do, forgetting those things which are behind and reaching forward to those things that are ahead, I press toward the goal for the prize of the upward call of God in Christ Jesus. Therefore let us, as many as are mature, have this mind.”
Dwelling on the past is of no help to anyone. We are to press forward toward the goal and not keep looking behind. Trusting God’s forgiveness through His grace, we’ll also have faith in God’s further grace of ultimate salvation in His family. So we strive for that goal, for the prize that God has offered to us as a free gift.