Forgive and Forget
In Isaiah chapter 44, we read where the prophet gave the Word of God concerning forgiveness. He said God not only forgives one's past sins but also forgets them to never be brought up again. He never will bring up someone's past but there is one that will. Satan is notorious for dredging up one's sins from his/her past and used it to discourage him/her from doing work for God. That was so satisfying for the devil, he didn't need to do anything else.
We need to follow God's example and not only forgive another for what they may have done to us, but also forget it. It's impossible to do it on our own power but with God, all things are possible. We can forgive the person who offends us and forget the sin as well through the Holy Spirit. We can do all things through Christ who strengthens us.