Most Helpful Girl
I just answered something similar to a Question like yours here, dear, and simply told the guy to talk to her face to face and lay it on the line That------I want to break up.
Be more subtle, more caring and diplomatic when you go about this. You have all the moves and grooves in what you want to say. However, being with this 'Chick for over a year,' I believe you should Word it so she doesn't balk and squawk right in front of you and it turns into a Huge fight.
She is a sensitive teen who most likely has feelings for you after all this time. Tell her you really have liked her and cared about her but you are not ready for a serious relationship right now and that it is not fair to her when she is a good catch for some other guy. Tell her you feel you think it is best that you both break up and go your own ways but you hope you can still be friends, if that it is okay..
Good luck. xx