I would write back when I was in HS.
The girl is just using you, and not for the stuff you want her to use you for. She won't date you or do anything with you, just move on. Don't let your gf push all your female friends away, and also stop caring so much about having a relationship or something and get laid.
Don't break up with your gf through the last year of HS. She will get you a good job, and help you learn things about yourself that you wouldn't have for a long time if ever. Once you have that job, end it. Immediately.
Use your money from your first job to buy stocks of Google. You will be much better off financially then if you go buy that really cool tablet pc.
Don't buy the gf a car. Or if you want to be stubborn and buy her one, spend $1000 on it instead of a brand new car. It will be a $5k mistake, that will affect you for a few years.
Just keep the girl that you meet through a friend and hang out with over the summer of 07 a FWB. Date her if you want, but it will not go anywhere. She will leave the concert with another dude, even though she says she loved you a day before. Don't get attached to her at all.
In your freshman year of college, go hit on more girls and goto parties.
Save more money, start saving it ASAP. Stop buying stupid things, and work on saving money.
Live life. Don't worry about people so much. You can only control yourself, and things you do. Don't get hung up over a girl, let her do what she will but enjoy your life. She was a b*tch anyways. Being single is fun, enjoy it. Live in the moment, and stop thinking everything out trying to analyze everything. It won't help. Learn to enjoy the simple things in life. Don't spend so much money on girls you like, because they won't appriciate it or were just using you anyways. A girl will like you for who you are, and buying her things is NOT the way to keep her or the message you want to send. Caring for girls and going out of your way for them on a regular basis will result in you getting walked all over.
Don't stop riding your bike, go skydiving sooner, get into paintball sooner, and don't let the long term relationship girl (who you shouldn't propose to because she isn't worth it) say you can't do something.