I would have prevented by parents from using psychoactive medicine on me. Even though I was very young and it could not be said to be a reasonable thing to deny your parents anything at that age I still should have been stronger. I later learned that it has implications for your development so might have changed who I am. And not for the better.

As a close second: telling girls I was interested was something I could not do for a long long time. It's still difficult and I know it's very common for men to have these sort of regrets but I managed to overcome at least some of that.

I could have studied harder too but it was very difficult for me.