How many times has someone said something to you in anger that you thought that person had forgotten about. You thought that apologies or actions would have been enough for that person to get over the hurt or sting of the situation. This is where we on either the receiving end or giving of the hurt or sting need to realize that if we do not forgive the person we will hold and harbor those feelings for a long time. It is especially harmful when these feelings are held in as you get older. The stress and strain on your body, mind and soul are unforgiving. It takes a toll. What it takes is to actually release that hurt. Release that pain. Let it come out. Forgive that person and understand that in different places and times the who what and where can be forgotten but the why is the thing that we hold on to. If that person is still on the planet it might be a good thing to ask them. If they aren't available perhaps write a letter, place it in a bottle, release it into the ocean. Record a message to that person. The best thing to do is to pray about it and give it to God, and never pick it back up again.
2 Corinthians 5:17
Therefore, if anyone is in Christ, he is a new creation; old things have passed away, behold, all things have become new.”