Guys, scrupulosity is a mental condition - I had it.
It’s like this: occasionally, you are genuinely unsure about a sin being mortal vs. venial. But the vast majority of the time, your rational mind tells you “I’m being silly” but you have niggling thoughts of “how can I be sure? what about this slight nuance? what was I really thinking when I did it?” and it won’t go away. It consumes you.
For me, rather than go to a priest, I used to go to my dad (I had it as a teen and he had it when he was younger too so he understood). When my dad would tell me “that isn’t a mortal sin” (or “that isn’t a venial sin”), I’d get a massive flood of relief and I’d be content until the next time I did something “wrong”, probably a day or two later. For me personally, I think it was a way of processing stress from exams and things like that.
Anyway, my point is, if you’ve never had it, it can seem crazy from the outside. But for me, I was really just looking for that flood of relief from the annoying niggling feeling - I just wanted to feel OK, to feel certain about something. There is nothing rational about it.