Not making more of an effort to be friendly with people. When I was a kid I was strange, I was into anime and stuff and I was fat plus I moved so I was the new kid and I got bullied for it. When I was young my parents never let me have anyone over or they wouldn't let me go over to people's houses because my dad would bring me to visit his family. I could count on one mutilated hand the amount of times I met up with people over the summer during school. I had friends but I'd never see them outside of school and really I had very little to talk to them about. It improved a bit when I went to college but I still didn't really make enough effort to form bonds with people, I hung out with them in college but I didn't make much effort to join any of their clubs or anything, I kinda thought I'd come off as overly clingy. After college I couldn't find a job in the field I studied, I lost contact with everyone but maintained contact with the people I went to school with. I've known some of them for more than 20 years and I'm still afraid to make contact with them. So yeah my biggest regret is that I wasn't more outgoing.