Whenever I start thinking about past experiences and people in my past that I know I need to continuously forgive, I turn to the Gospel. The Church has wonderful videos that can help us through those tough times, if we let them. This video is one of my favorites on forgiveness. If he can forgive this young man, I can surely forgive people in my past.
When we don't forgive, we are essentially telling Christ that His Atonement isn't good enough for that person. We have to forgive. It's as simple as that. And I know that when we do, our lives are filled with peace.