I'm having hard time relating; my experience is literally the opposite of yours. Grade 7-12 (if I parse that correctly; I mean 12-17yo) were literally the best part of my life. School was boring AF, but had its moments; it gave me some interesting people to talk with every day, and then I had plenty of time to pursue my interests. Stress? What stress?
Before that period? I don't remember much. After that period? Everything gets downhill. Being an adult in a society sucks and I hate it. Normal people lifestyle, with its routines and "work/life balance", with everyone around you having expectations around random customs, with having to do everything in tiny bite-sized chunks, because there's not enough time left after work, chores and family - that's just not compatible with my mind.
Yeah, I guess, this could be a mental health issue.
There was another discussion @hn a few days ago where this was explained. When you're young, your life is pretty much on rails and you are very sure what the next step is (ie. kindergarten - > school -> high school -> uni). But when you finally finish your education, the tracks end there and now you have to figure out where to go from here and that can be mentally draining.