Why regret? One cannot change the past, nor should you. There are things I'd done that I am not overly thrilled about, but change any one of those things and I would not be who I am at this moment. Had I been more popular in school, I'd have never met my best friend. If I never wasted two and a half years in college for a near-useless diploma, I'd also not taken the pizzaria job where I met another good friend, then later moved on to my current job and thus bought my nice Matrix. And as much as I may pine for this girl I once knew, wondering if we could have been something together, things could have also gone sour, leaving me with bad memories of her instead of good, or worse yet, a child I didn't want at the time.
If you can be at least marginally happy with your life as it is now, there is nothing gained by regrets. Things could always have gone better, yes, but they could also have been worse. Who knows what one little change you make, could have actually been the mis-step that cut your life short? Or driven you to bankruptcy?
Edit: You have NO idea how hard it was, upon seeing this thread's title, not to come in here and simply post "Your Mom!"...
. Me on the otherhand I go into my doctor, knees a popping, and he says "I don't see anything wrong." Yeah that's why its excrusiatingly painful to lift anything much higher than my body weight with my legs 
