I have a lot of regrets, but when I think of the biggest, I come back to how I treated my parents in the late '90s. It was a combination of what I call my "OCD focus" starting to take hold, immaturity, social issues, and just being a jerk. I didn't actually feel or believe that they deserved the way I treated and talked to them, but the way I was doing things and why I was doing them fit my OCD issues (I wouldn't say this was classic or stereotypical OCD, but it's what I call it). They didn't deserve anything of what I did or said. It came to a boil-over on Christmas Eve of 1998. I was a total, sullen jerk. I recall my mom pleading, almost tearfully, to my dad and brother not to let my behavior ruin our Christmas.
In the years after we repaired our relationship, but to me, it doesn't matter if I'm "forgiven" to them. I still carry around the regret and self-hatred for how I behaved.
Socialism is the philosophy of failure, the creed of ignorance, and the gospel of envy. ~Winston Churchill