This is actually such a difficult question to answer... because changing one thing in the past could change other things. If it had to be one thing, I'd go back 15 years to my 12yo self and say:

Your parents are wrong. About what's right and wrong, how they treat each other, how they treat you, and who you are. You're not an awful person; you try harder to do the right thing and to care about those around you than a lot of people. Yes, you're smart, and yes, you're capable of a lot, but no, that doesn't mean you're perfect nor should you be expected to be perfect. Mistakes happen, things will fuck up, that doesn't make you less of a person. Aim for perfection, but accept 90%. You'll still be 30% higher than most people you'll bump into...and your confidence will be in your ability to succeed... not in your success. That's important, because you're name is Racer, so life is going to hate you, a lot. Things will go wrong when there's no way they should, and despite that, you'll stay strong for a long time, because you're a stubborn and determined person, but unless you learn where to put your confidence, you will eventually break, and life really gets hard then.

Oh, and for fuck's sake, actually go for Molly or Brittiney and fuck their brains out instead of being the awkward little shit that let everything go by him. You'll regret it. A lot.