I think I would change the wording on that slightly. My past COULD have been different: my parents could have died and I could have been raised by someone else; I could have succeeded in my attempt to run away at age 14; I could have succeeded in the multiple suicide attempts.
But my past wasn't different, and as I don't own a time machine, my past simply cannot be different.
I would say "Forgiveness is accepting that the past was what it was, and that we have no way of changing what has happened in our lives previously. All we can do is change what is happening now and in the future."