Here are a few thoughts for consideration.
1. The Bible does not address the problem. If it were real, the Bible would speak to it.
2. The Bible speaks about being forgiven and forgiving others.
For if you forgive men when they sin against you, your heavenly Father will also forgive you. Matthew 6:14 (NIV)
This is how my heavenly Father will treat each of you unless you forgive your brother from your heart. Matthew 18:35 (NIV)
Be kind and compassionate to one another, forgiving each other, just as in Christ God forgave you. Ephesians 4:32
3. The Bible speaks of two kinds of sorrow, Godly sorrow and worldly sorrow.
Godly sorrow brings repentance that leads to salvation and leaves no regret, but worldly sorrow brings death. 2 Corinthians 7:10 (NIV)
Not forgiving oneself is worldly sorrow. It results in death. Godly sorrow leaves no regret. Not forgiving oneself is living in regret, which means it wasn’t Godly sorrow, which means God has not forgiven because the sorrow wasn’t Godly. In other words there is no such thing as being forgiven by God and not forgiving oneself. If forgiven by God there would be joy.
4. The person who says he “cannot forgive himself” is making himself senior to God. Who does he think he is?
5. There is another problem; that is in his use of words. He says he “cannot forgive.” He means he “will not forgive.”
6. This is not a virtue. It is denying the work of Christ, the faithfulness of Christ and the word of Christ.
But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus, his Son, purifies us from all sin. 1 John 1:7 (NIV)
If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness. 1 John 1:9
7. This saying “cannot forgive” is one major sin that needs repentance and forgiveness.