The only major thing I really regret is my meltdown some years ago.
It was unavoidable, really, in some respects. I had been under a lot of stress from high school and I was under a deep depressive and introverted streak, keeping all this bottled up inside. And then my girlfriend dumped me. That was the final straw and I went a bit nuts.
Looking back on it, I was a real dick to a lot of people and probably lost a few good friends in the process. I will always regret my behavior at that time, but I look back on it as a reminder of why I shouldn’t let myself get all stressed out like that and why it’s important to have people to talk to. Some good came out of it as well. The relationship I was in wasn’t really a healthy one and while it hurt to lose her, I think that was good for me in the long run. I also made one hell of a good friend in the process (she and I have been friends now for something like thirteen years) and learned a lot about myself as well.
Still… I kind of wish I could round all the people I was a jerk to up and apologize to them. Oh well.