It's per required need. Like a benzo I suppose.
I can relate Outlaw My pdoc calls it catastrophic thinking - I should ask her more about why she thinks I do it. She just tells me, Got to watch out for that catastrophic thinking.
I don't know why it happens. Could it have to do with when you almost left her? And coming to terms with that? Like not trusting yourself because of what we did when manic. Idk, and maybe I am totally out of line. I don't know why I do it either. Usually has to do with my kids, or my mom. When I was a kid & terrified of my mom leaving the house, I told her Please don't faint or fall down! I was totally serious. She reassured me. Somehow now that I have some space from that feeling, it's now a cute joke we say to each other getting off the phone. She's shared her ocd tendencies with me & understands.

Regarding taking a prn, I don't see how a pill could stop those thoughts, maybe for awhile if it makes you fall asleep. But for me, I end up having nightmares about the thing. I think talk therapy or the type the others above say, is the best bet - or talking to us here about it. Talk it through in as many circles and loops as it takes, where you get to using the logical side & calm your brain down. I did learn some grounding techniques that helped. Sometimes I just pace & pace & that sucks.