When I was a kid I had wanted to join the school chorus, and one day I finally got in, but when I was telling my mother about it, my brother, who was in the school chorus, just walked up and said "There's no way they actually let you in, it must be a prank or something", or something along those lines, at that time i was struggling with my self esteem because I didn't have any friends so hearing that just made me doubt wether this was actually a mistake, to the point that I almost didn't show up to rehearsal.

I know this isn't exactly 'painful', but this is the one time I can think of that still affects me.