1) I'd have said no to that position in a social club back in high school, rather than worry about not being able to do what was expected, not even agree with what was expected, and then simply not do it.
2) Thought before speaking and thus avoided unintentionally insulting a high school classmate.
3) Not threatened to chuck out DDs comics if she didn't clean them up by deadline, so that I then wouldn't have had to follow through on the threat.
4) Taken DS Iceland pony riding that one day on Iceland, rather than joining the rest of the gang (two-family trip) sightseeing in town.

Other than that, I don't really have a lot of regrets. Sure, I could have done lots of things different or better, but I don't think I've done them so badly as to be hurtfull so I think I should just accept my weaknesses and move on.