I think about this a lot, but then again, I've already changed some things in my life:
I wouldn't have gone to college straight out of school.. I wouldn't have worked my but off in high-school only to smoke too much weed and drink too much beer in college.
I would have told her how I felt/feel rather then letting her run off to Africa.
I would have seen my parents divorce coming 10 years ago and persuaded them to go through with it then.
But hey, things happen for a reason. I started in pre-med 6 years ago in school, graduated with a BA in anthropology, and am now going back for graduate work in Nursing next month. You just have to make your mind up and go with it, and not think about the 'what-ifs'