"Forgiveness is letting go of the hope
that the past can be changed."
-- Oprah Winfrey
When you think about people who are holding on to a grudge or thoughts of past wrongs, etc., it really puts it into perspective that somehow they are hoping and wanting somehow that the past can be changed. If only that other person would do amends, right the wrongs, admit their errors, we could maybe get past this place. But, we all seem to know that those things won't change the past and we even say it that way.
Of course, they can't change the past, but we certainly can change today, this moment, and one of the most effective ways to do that would be letting go of the hope that the past can be changed. Once we let go of that hope--even by seeing that this is a hope--we can perhaps just let it rest in peace and move back into the present.
Besides, dealing with all that past in the present just stops us from living. Think of a rope and the more it is tugged on, the more the tugging back. Now, drop the rope and walk away. There is peace right there. That is an act of peace.