I think it is valuable to examine this idea as it occurs in reality before God. This is what I have witnessed of people being freed from this: “I just can't forgive my self”. They have experienced it in varying degrees. Some hounded by it, others returning to the “re-experience of guilt” just periodically (“a wave of guilt” returning apropos of nothing). Some never having experienced the forgiveness of God on the issue, eventually came up with the idea it must that they have not forgiven themselves. It has also become a popular idea in “Christian psychology” circles. I will list what I have learned from examples of people that got this resolved.
“I just can't forgive my self”: You are correct. You cannot forgive yourself. Only God can forgive. We can seek/receive the forgiveness of God for sin when the Holy Spirit has convicted us of SIN (forgiveness is with respect to sin alone). You can seek reconciliation with the one you sinned against. They may or may not be able to forgive you (their level of maturity/Christ-likeness). When the Holy Spirit has directed you to seek reconciliation and you have obeyed in identifying your sin and asking their forgiveness, their forgiveness of you is between them and God alone.
How is it that you cannot forgive yourself regarding one specific sin, while that was not a problem with so many others? That should be a tip-off something is askew.
So whatever can/does it mean that when God has forgiven us of SIN, and we say or experience “I just can't forgive my self”? From what I have witnessed it is worth examining ourselves in these circumstances. Do you have a higher standard than God? Who has the authority and power to forgive sin. What does it mean to seek forgiveness from self? Exactly how likely is the religious old man to forgive you, let alone be able to (this is a renewed mind issue). Put another way: Who is the “I” in that sentence and who is the “self,”
Did God direct reconciliation/restitution which was not obeyed (saying no to God, pride interfering with humility, etc. which effects our experience and can confuse)? How clear and specific was the conviction of sin AS SIN?
This can be a faith/doubt issue involved regarding God's forgiveness. How far is the east from the west if we can hold onto what God has cast away. Was there thanking God for his forgiveness (thanking is the first step of faith when forgiveness, or anything, is received). We should not forget discerning the accuser of the brethren and the shield of faith.
Once in a case of forgiveness regarding abortion, there was a confusion of guilt regarding not giving testimony of forgiveness blocking/refusing God's working together all things for good (Romans 8:28). After testimony, a wonderful ministry opened to this woman. Her experience of “guilt” had nothing to do with “forgiving herself.”
I can recall one unusual example in which the person was often reminded of their having not forgiven another, in the midst of repeatedly seeking forgiveness for their sin. They had repeated experience of receiving God's forgiveness (or comfort regarding that) but it seemed temporary as the experience of guilt returned. When they dealt with their un-forgiveness as sin, the issue was settled. I thought this was a wonderful example of God teaching Mathew 6: 14-15 to a “stubborn” child of God.
[b]I am persuaded by the Word of God and by the works of God I have seen that the forgiveness of God is complete.[/b] I think every case is extremely specific and revealable only by the Holy Spirit of God to the individual. Things I have seen involved include growth in Him, continued renewing the mind on some specific Truth, and greater freedom from bondage. Is this not always the case with us?
I know others like Twayneb posted similar ideas, and as I post this I see there are many other posts here that I have not read. These come from testimony. Hope they help.