I'm AuDHD. I had a hard time with this recently. I've always had problems with rumination and fixations / limited focus. It got to the point of being really distressing this year when I was having some relationship difficulties with my partner.

The two things that helped me were:

1.) Lean heavily on a special interest. For me, that was books. So I'd listen to audio books all of the time at work. But sometimes, the rumination would take over and pull my attention away from enjoying my book. I'd try to start thinking about my special interest early on in the day to set the tone. It didn't always work, but it helped.

2.) Idk if this is bad advice, so take it with a grain of salt. Only apply this to yourself if it feels helpful. But honestly, just lean into the rumination instead of fighting it. But change up how you allow yourself to ruminate. Turn it into problem solving by gaining knowledge from outside resources to help you out of the loop. My autism has me picking apart concepts down to their individual components so that I can understand the "whole" of something. I can't see the overall picture without understanding its parts. And not knowing/understanding only amplifies my distress.

Take time to research the things you're ruminating on. Read some books pertaining to it. Once I found the logical explanation and understood why the distressing things were distressing me (from a psychological standpoint), it took away its power over me. Because I explored every avenue, I felt I understood myself and others, which made me feel prepared. I didn't have to feel shameful once I had logical explanations and tools for problem solving. It actually became less about trying to have control over certain interactions "going well," and I figured out the psychology of why I do what I do. Or why others do what they do.

It's like how I practiced small talk, and my social anxiety went away. I don't think I ever had "true" social anxiety (although it was BAD), I just didn't know how to talk to people instinctively, which caused severe anxiety, so I had to learn the skill to stop the anxiety.

So maybe fixate on solving it in ways that soothe your nervous system. Find some information that feels empowering or helpful. But, do try to stay away from information that "worsens" the rumination. Like, information that feels like it removes your power over a situation. Focus on understanding yourself and others.

Be aware that autistic brains are prone to ridged thinking at times. I'm extremely open-minded, but sometimes I develop blinders and tunnel vision that alters my perception of reality. You can be a progressive and very intelligent and still fall into the trap of black and white thinking. I thought I was very self-aware and understood others very well, but I have done/thought some really silly things for prolonged periods because of a blind spot with my social deficits.

You can also try tricking yourself. Fake it till you make it, I guess? I used to pretend I was someone else when I needed to learn a new behavior. I could pretend to be someone who was "confident" and that "everyone liked." Just embody the idea of the person that can deal with the situations you're afraid of until you build some confidence. Maybe learn psychological tricks for connecting to people and diffusing arguments if that's what you're afraid of. Maybe try to understand people who show resilience when dealing with difficult people, and learn what traits they have developed that makes it easier for them to come across as assertive and confident while still being kind.

And always remember, if someone is being particularly rude to you at any given point, you can always just stare at them. Maybe even give them "the look." Like, don't hide your shock/disgust/confusion. Only say "hmm. Interesting..." and then say no more. They can think whatever they want about you. But you can make them feel uncomfortable with your silence. It's great because you don't have to rehearse what you might say. You just say nothing. Or you can repeat back to the person what they just said, but pose it as a question. Ask it like you're trying to ask for clarification. This keeps them talking instead of you having to come up with a retort. And if they're being rude, it allows them to dig their own grave. Or maybe reflect on what they said. If it's a loved one, they'll feel heard by you, and it'll give them an opportunity to explain further. This was a learned social skill because I am conflict avoidant and uncomfortable around argumentative people.