Well, you had to do what you had to do, right? Think about having to do that another year down the line, when she has probably gotten a lot more attached to you.
As for staying friends, it's a tough matter. After crossing the lines from friend to boyfriend and girlfriend, it's hard to go back. Of course you can still be friends, but there will probably be something different. I don't know why... I think it's just something that comes from the fact that you are now an 'ex'... even though you may be best friends. Just keep in mind that in most cases, you don't end up staying friends. But no matter what you do, don't become an asshole to her (which I'm not worried about)... and don't be too surprised if you two kinda fall out of the loop with each other. Remember, this is a risk that both of you had to take and maybe knew you had to take when you committed yourselves to a relationship.
As for college... I will be a senior in high school this year, so I can imagine how that is. I feel that I have separation anxiety and am very afraid of what's coming after high school. Right now I live in southern California, and want to go to Berkeley. All of my friends are either going to stay around here or go to Florida... so I think it's going to be pretty rough on me. But I guess people aren't meant to be together forever? So I guess that theoretically, I know how you feel right now (even though I haven't experienced it first-hand).
And you're right. Life goes on.
Best of luck to you.
-Prodigal
P.S. Have you read my thread yet?
If a dream comes true... then is it still a dream?