I would have changed nothing. Why?
Well for one despite the hell I had to go through for most of my childhood and teenage years, it made me a better and stronger person today. Yeah, I'm a bit messed up because of certain situations but hey, if I can live that, I can live through anything life tries to throw at me.
Besides, I try to live a policy of no regrets. If I screw up, I screw up and live with whatever reprocussions may come of it. Things happen for a reason and usually everything works out in the end.
Also, techincally if you build a time machine today, you can't go back to yesterday because the time machine did not exist yesterday. Since it didn't exist then, you can't go BACK into a time before the time machine was created. It's some kind of parodox or something. There's a whole discovery channel series on it on youtube somewhere.