I don't mean just moving on from LVM, even low-value acquaintances or colleagues.
In a situation where someone has been extraordinarily rude or has wronged you, and you were not able to get satisfying closure, how do you stop ruminating on it and stop letting it bother you?
Journaling, and letting myself be really fucking pissed about it. Confirming my right to feel unfairly treated and not looking to others to give me permission to be angry. We ruminate on feelings endlessly because we feel like we might be missing something and need to analyze further. No, sometimes things are very simple and clear cut and we just don't like the answer (because we don't want to believe that someone is really an asshole, for example, or because we dread having to enforce boundaries). By accepting the answer and drawing necessary consequences, it's easier to move on. Often we regret not having reacted "better" and made our boundaries clear, we have to move through that as well and have a clear plan for what we will do next time someone treats us that way. So yeah, acknowledging our feelings and letting the pain, anger and regret just be and be thankful that these emotions are there to protect and guide us. They wouldn't be there if they didn't teach you something about how you want to be treated! Thank your body for looking out for you. Thank yourself for actively moving through this instead of pushing it aside. And then realize that these feelings are not forever and actually fade away sooner when you have welcomed them and listened to what they had to say. Talking to others is helpful initially, but to really move on, you have to talk to yourself. We all look to others to soothe us and give us insight we lack, but everything you need is already inside of you.
Get busy
Look up cord cutting.
I give them funny, silly or insulting names. And make up funny reasons as to why they acted that way. It can work for anything but my friends and I have names especially for our exes- like Smelly Sock, Little Linguine and Burger Boi (those names are based off stupid things they've done). The fact there’s no closure doesn’t bother us anymore because those names reminds us of their stupidity.
Try to detach your self-worth from whatever happened.
Acknowledge that they are miserable fucked up people who will suffer the worst punishment of all; they have to keep on living as themselves. But also what @PlainSimpleTailor said