Warnings: Contains mentions of violence and death.

I feel trapped in my own mind because I can't stop thinking about someone I don't want to think about. To make things simple, the facts are these:

• A few years ago, a person I genuinely thought was a friend found out I had started dating someone else, and as a result, we are no longer friends. The falling-out lasted over a week when all was said and done, and they sent a large number of hurtful emails during that time. Among the things that still stay in my head was the line "you have been a terrible friend" and another friend telling me when I expressed a desire to try to repair the friendship, that this person was "dangerous" and "not someone [I] want in my life."
• [Literal years pass]
• A couple months ago, I saw a post on reddit from a woman describing an almost-identical falling out she had with one of her (male) friends and it scared me and brought all those memories back. I realized what was about to happen, was able to ground myself, and move on until....
• The very next day I saw a news article about a man who had killed his ex-wife 4 years after they were separated. That's when my mind just took this idea and ran with it. I was so SO scared, thinking that this person was going to show up out of the blue one day and hurt me and because of the actual events that lead to my diagnosis I was convinced that that was a real thing that would happen.
• I ended up actually looking up that person online to make sure they weren't physically nearby anymore, and that helped with the fear, but now I'm stuck with a fixation that won't go away.

The cycle goes like this:

Finally stop thinking about it -> Realize I'm not thinking about it -> Be happy about that fact up until I realize I'm thinking about it again

I also feel terribly ashamed when I have these thoughts around the person I love, because I feel like I'm being disrespectful to them by thinking of this other person while I'm around them.

I'm not doing so well and could really appreciate some advice. In the last two weeks I've even taken down some of the art in my house (art I really like!) because it reminded me of this person (just slightly) and I even considered writing them a letter to apologize for the things they accused me of being, because I think in some way self-flagellation will protect me or make this go away.

Any advice on helping myself get through this and out of this headspace would be greatly appreciated.