The only way to overcome unforgiveness is to base your forgiveness on faith in God rather than feelings. True forgiveness does not have anything at all to do with how you feel. It is an ACT OF THE WILL.
That means once you have forgiven the person, you need to consider them permanently forgiven! When old feelings rise up within you and satan tries to convince you that you have not really forgiven them, resist him. Say, "I have already forgiven that person by faith and I refuse to dwell on those old feelings."
In 1 John 1:9 it says "If we confess our sins, God is faithful and just and will forgive us our sins and purify us from all unrighteousness."
Then acc to the above vs, believe that you receive forgiveness and cleansing from the sin of unforgiveness and unrighteousness associated with it - including any remembrance of having been wronged.
Looking at my mother-in-law now, frail and vulnerable as she is, I'm glad I had forgiven her years ago for her hypocritical treatment towards me. I realised later she held that attitude towards all her daughters-in-law. Unfortunately, she was also dealing with feelings of self rejection, unforgiveness and fear in her own marriage. As I choose to forgive, the anger associated with unforgiveness disappears.
Is there someone whom you need to forgive? Perhaps a close family member or a friend's betrayal? Someone whom you trusted had sold you out? A boy/girl relationship that had gone sour?
Peter and I will be meeting up with a couple whose 3-year marriage has gone awry. The husband cannot forgive his wife for double dating during their courtship. Although that happened 4 years ago, in his heart he could not forgive her. This year, he finally walked out because he could not carry the burden of unforgiveness any longer. The real victim is the wife who believed that the man who married her loved her. It was selfish love, a love that keeps records of wrongs!
The choice to forgive is a deliberate and purposeful act. It takes everyone off the hook. It could set you free from years of imprisonment. Also, releasing those who have wronged us is fundamental to receiving from God.
Mark 11:24-25 " ... whatever you ask for in prayer, believe that you have received it and it will be yours. And when you stand praying, if you hold anything against anyone, forgive him, so that your Father in heaven may forgive you your sins."