I call this "looping." The thought loops continuously. A high dose SSRI can help but the side-effects are not worth it.

I have songs I use to replace the looping thought. When I get stuck looping, I start singing a song in my head. The song needs to be something that is not a problem if it starts looping. Songs with repetitive lyrics should be avoided. A poem could also work.

btw, The psychological term for this is sometimes 'perseverance.'