Welcome to PC!

I have had a very similar experience except the fading of the obsession came about after I stopped using stimulants but I think said stimulants were probably just magnifying the underlying illness.

I'm not sure but I think they call this a delusion of infidelity? Or morbid jealously. Google it, I could tick every single symptom off the list, was odd when I found this out AFTER all the jealously dried up. Of course I was convinced of my beliefs against all actual evidence at the time, a whole 3 years of time, it was only after I stopped using that I questioned my behaviour & thoughts.

I'm fairly sure amohetamines in particular are related to morbid jealously. I realise my situation is different to yours as mine was brought on by substance abuse but mania is kind of like being on stimulants so maybe it's got something to do with dopamine? Were you experiencing any specific mood episode when you had these thoughts or was it just something you always had in your mind? Mine was kind of always there just worse when using, that's when I went full private detective haha