If I were 12 again, I would probably throw myself under a truck as well. I wouldn't want to go through all that sh*t again. I'm happy where I am But, if this really did happen, I would probably work on trying to prevent 9/11. Maybe get onto one of the planes that was hijacked and try to stop them. On the years leading up to 2001, I would work out like all hell so I could go onto the plane and kick the crap out of those b*stards.
But then again, you would have to bring up the question of whether or not changing the past would be a wise decision. Perhaps all things happen for a reason, and even the most atrocious events must be left to occur. But even so, I would not let something like that happen if I could.