Well, don't know what my diagnosis might be. (One psychiatrist I saw said she thought there might be a "bipolar element" to what was going on with me... whatever that means.) But what you wrote about getting stuck on things certainly rings a bell.

One time / place in particular where it occurs is in the morning when I'm taking my shower & then drying the shower down afterword. This whole process takes a while & I always find myself stuck on some thought or other. Typically it's something from my past I just can't let go of. But occasionally it will be something based on some tidbit of current events I just happened to stumble upon. (I try to avoid exposure to the "news" as much as possible. But occasionally some of it sneaks through regardless.) I suspect if I wasn't as reclusive a person as I am, & was out-&-about in the world, it would occur more.