I wish I laid off the booze and drugs when I younger. The $$ spent on that if it was invested instead.....[facepalm]
Other than that, I figure that almost everything I did in my life lead me to my wife (we married in 2011 when I was 41yo) so I have no regrets. I do think "if I only did this or that..." sometimes but I always come back to the fact that I wouldn't have met the most wonderful woman in the world if I did things differently.