This is a great question that I bring up to people/myself from time to time...and a tough one. It's easy to say of course I would, things would be so much better. I wouldn't make the same mistakes that I did in the past, but I just don't think it works that way.
It's sort of a contradiction, not to mention a problem with the space-time continuum ;-) Honestly, the only reason you're the person you are right now is because of the mistakes you've made in your past. It's hard to appreciate the sun without the rain. I hope that wasn't total nonsense.