I don't know if that's unusual. In highschool you start making your first real decisions, as opposed decisions being made for you.
I'm not happy with who I was during highschool, I did not make friends, didn't learn usefull skills outside of the classroom (and not many inside of it), didn't exercise a lot. And I went to a perfectly fine highschool, so the blame is entirely on me. I can still get frustrated at myself for the waste of opportunities.
I don't know how helpfull this advice is, but I'd say try not to get bothered by it. Yeah you've made mistakes in highschool, I'm pretty sure we all did. It's good to reflect on your mistakes, and try to learn from them. But at a certain point you just got to let the past go. You say that you go weeks or months without caring, that sounds like the problem is very managable. I don't know how long those obsessions last, and what you do during them. Assuming that they aren't that long lasting, and you don't do anything extreme I wouldn't say it is a real problem.
Allow your brain to get stuck on something silly sometimes.
So yes I can relate to your obsession. But based on the picture you paint here I wouldn't bother trying to resolve it. But I've the luxery/problem to be generally aphathetic, so perhaps it's easier for me than you to let some things just go.