You're in a situation that most of us go through. It is tough and it is hard to get over, but it has to be done. I honestly think you should let her go because if you don't, she's going to let you go and you're going to be crushed at the end. You have to realize that she is going to college and having someone that is still in high school is going to be a big burden on her, especially since she's going to wanna explore and live the college life. It'll make her feel as if she's trapped.

What I think you should do is let her go and become friends in the end. Like GrazyLovesMari said. It's her future...