The mercy of God is really impossible to understand. Trying to contemplate how it is that a God of justice and perfection could forgive sinners is well beyond our reasoning capacity. Many people who I counsel, however, tell me that they have more trouble forgiving themselves than they do accepting God’s forgiveness. But what really lies behind this idea of “I can’t forgive myself”? The language of “I can’t forgive myself” actually reveals a person’s pride.
A preoccupation with self-forgiveness really reveals a heart that believes our judgment is more serious than God’s. When we say “I know God has forgiven me, but I just can’t forgive myself” we are acting as if our self-judgment holds more weight than God’s. Can my opinion of myself overrule God’s acceptance of me? Can my view of myself carry more weight than God’s view? When I say “I can’t forgive myself” I am acting that way.
It also reveals idolatry in our hearts. Tim Keller is says:
When people say, “I know God forgives me, but I can’t forgive myself,” they mean that they have failed an idol, whose approval is more important to them than God’s. (Counterfeit Gods, 172)
Idolatry in our hearts says that it doesn’t matter if God forgives me. There is still an ideal, a standard, that we haven’t lived up to and we are disappointed with ourselves. His forgiveness means less to us than the realization of that ideal. Perfectionism in people is really a symptom of idolatry that says the standard I have set for myself and failed to obtain is the most important standard. Experienced counselor David Powlison points out that often remorse is not a sorrow over sin as God sees it, but sorrow “against their idealized self-image, a remorse in their own eyes, and a remorse against what other people think about them” (Quoted in Redemption by Mike Wilkerson).
The struggle with self-forgiveness also assumes that our sin is more of an offense to us than it is to God. In making self-forgiveness so crucial to our progress we act as if it is an easy thing for God to forgive. Our standards, however, are higher and it is harder to find self-forgiveness. Mike Wilkerson writes, “It is the height of self-centeredness to think your sin somehow offends you (or anyone else, even) more than it offends God” (Redemption). Your sin offends God infinitely more than it could ever offend you or anyone else, this is why Jesus’s death was necessary. God’s forgiveness of you has not come easy, it cost him greatly! To suggest that self-forgiveness is more important is to belittle the sacrifice of Christ and reveal your own pride.
A heart focused on self-forgiveness may also give you a sense of control. Feeling like you have the final say in your guilt may give you a sense of control. Performing acts of penance to try to satisfy your own sense of justice may even make you feel noble and respectable. God has dealt with your guilt in Jesus and assured you of forgiveness when you repent, but by maintaining that you still owe a self-debt you give yourself a sense of authority and control over your situation. This is a deep pride that will keep you from true repentance and true progress.
When the Bible speaks of guilt it speaks of our guilt before God. We may need to repent to others of our sins, but ultimately if we know God has forgiven us we can stand free from condemnation (Rom. 8:1). Self-forgiveness is not the Bible’s method of dealing with sin. Be careful of pride in your heart, friends. Let God’s forgiveness be ultimate in your life.
Sparrow & Heart Soul Care 
Very challenging and thought provoking but i would argue that fear is a big cause of not forgiving ourselves. It’s so hard to believe that we can do such awful things and God can still love us and forgive us. I can make fun of people in a way that stays with them and God forgives. Someone can repeatedly rape and torture a child and God forgives. I am unable to wrap my mind and emotions around this and in trusting my feelings and mind their is fear. I guess in this case you might say fear is the idol. I would say fear is the natural reaction for us as children adopted into God’s family. In society, family, school and work we have seen both forgiveness and unforgiveness and we have felt for our selves the weight of trying to forgive someone who has done something awful to us. Knowing all this it is not first nature for us to rest assured in God’s forgiveness. It is something that must be trusted and practiced and believed when we hear his whispers to our ashamed and frightened hearts. To believe that we are safe in Gods love and forgiveness after growing up in a world the opposite of that takes practice. Luckily our God is faithful and patient and I believe he will keep whispering and loving until he feels our souls relax into trusting fully the promise of his forgiveness like a child who finally knows they are safe and home.