We make forgiveness complicated. I am, and have been guilty of this and when I embrace the simplicity of what it means to be forgiven I am always amazed by the reality of how far reaching it is in my life.
"God forgive me a sinner"
This simple statement, when spoken from the heart, rings so loud in eternity that if it were audible, the sound would deafen the world.
Forgiveness can be a process, it can take time to completely forgive someone for an offence. The key to getting free from carrying a grudge and reliving the events that have caused bitterness and stagnation in your faith walk is to continually allow God to work in this area of your life. It sounds simple because it is. Some people don't want to forgive. I have experienced this issue in my own life and in the middle of it God showed me that some times forgiveness happens in steps.
When issues like this come up in my daily thought life I give it to God (it doesn't have to feel all warm and fuzzy). And I keep doing so until that issue doesn't surface any more. When you do this your giving God access to your heart and the freedom to change your mind and free you up from oppressive garbage that may be dogging you in your waking hours. Un-forgiveness steals sleep and causes stress. It's distracting and destructive.
A pastor friend of mine counseled a woman who had been in and out of mental institutions for quite some time. She would shake uncontrollably and display emotional anguish that required heavy doses of medication to calm her down or enable her to sleep. They were about to commit this pour woman to a mental facility for permanent mental care. As my friend counseled her he realized she had been traumatized as a little girl. The long and short of it is she began forgiving the adults that had abused her. All the symptoms I mentioned above ceased. She was released a month later and never returned to the hospital.
See Mathew 18:22