I don't think I've managed to forgive myself for so many things, usually done during a manic phase. Ruminating is worst when I'm depressed.

Logic helps, like using a catastrophe scale (rate the event between 1-100, where 1 is doesn't matter and 100 is death), but its hard to do logic when your feeling like poop.

Destraction is probably the most effective tool I use. That and avoiding triggers that leads to the rumination starting.