Breaking up with a first love has happened to everyone, except ppl who actually marry their gf. Be as honest and caring as you can. It won't be easy, but in the long run she will get over it.

I've always thought that there should be a law against love before age 21. Maybe a maximum time limit (like 1 year) for ppl in relationships. This girl may be the right person for you, but both of you need the opportunity to explore other relationships to know for sure.