Unforgiveness is a high sense of self abuse.
You have to let it go, forgive yourself and forgive others. There's no other way.
If you're a person who can't forgive, there's something seriously wrong. If you can't apologize to yourself and recognize your faults and then move on, you'll be stuck forever. You'll never find happiness within yourself and in the universe.
Forgiveness is so important and beautiful.
I may not always "teach the person who hurt me a lesson" because I'm quick to forgive and I don't ever put someone through the "you don't know what you've got until it's gone," but I'm SO grateful for my gift of forgiving and forgiveness. For being mature and loving enough to let go of the pain and seek happiness, peace, and truth in my relationships. If it's meaningful, I won't let go. I'm grateful that I can forgive in the same capacity that God forgives me.
However, I need to forgive myself. That's who I need to forgive. Once I do that, I'll rule the world.