The one thing that I would change, if I could go back in time, would be to smack 20-year-old me and say, "Don't drink. Ever."
I mean, I'm very grateful for my recovery and I've learned a lot about myself through that process, but during the time I was a raging alcoholic, I hurt a lot of people, did some really bad things and only barely missed doing permanent damage to my career due to the generosity and kindness of some people who really didn't owe me that kindness.
I'm happy where I'm at right now, generally speaking, but I'll always regret the six or so years I completely wasted, and the pain I caused to certain people.