Holding a grudge is choosing to keep paying for something that already happened.

The original injury charged you once, and the grudge sets up a standing payment. Going back over what somebody did gives it your attention and your evenings, long after the person has stopped taking anything from you. Letting it go is not the same as deciding it was fine. It is deciding you would rather have the hours back.

If something is still running in your head, write what it actually cost you and what carrying it costs you now. You do not have to forgive anybody on a schedule. Just notice when you are rehearsing it, and put your attention somewhere it pays you back.