Ask: What can be learned? (Learn it, then move on.)
I find this to be so difficult though, because it is rarely the case that the answer presents itself lickety split. It's usually days, months or even years later that I feel like I have learned the lessons from it. The emotional distance gained from temporal distance is what allows me to hone in on the answer. I wouldn't be mulling over the past if it wasn't useful to some degree.