A grudge is homework you keep doing for a class that ended years ago.

Resentment feels like unfinished business, which is why it keeps demanding attention long after there is anything to be done. You go over the evidence, sharpen the argument, prepare for a confrontation that will never be scheduled. The effort is genuine and it is being spent on something with no remaining deadline.

When you catch yourself preparing the case again, ask what outcome the work is aiming at. If there is no realistic one, that is your answer. You are allowed to stop studying for an exam nobody is going to set.