When you dwell on an unpleasant past and mentally replay it over and over again, energy is wasted. The past cannot be rewritten, but mental chewing can be stopped with the tough pragmatism of awareness.
Ask yourself just three questions to regain control:
1. Can I change what has already happened?
If not, any regrets are a fight in reality where you will obviously lose.
2. Is there anything that can be fixed now?
If there is a genuine step or apology left, take it. If not, close the gestalt.
3. What conclusion do I draw for the future so as not to step on the same rake?
Turn pain into a dry and instructive experience.
The energy of focus is only needed where you can still act - in the present moment. Shift the attention switch to the "here and now".