I can't bear to break my gf's heart by ending it with her, how do I do it? Please read description!?

Wow I'm sorry you're in such a difficult situation. I don't know that I will have any good advice but I will try. First off, good job on being a man and at least trying to make this to where it could possibly hurt her a little less. Breakups suck.. a lot. Especially if she is convinced she is in love with you. All you can do is sit her down and be completely honest with her. Don't hold back any emotions. Tell her you still care about her as a person and wish her the best in life. Tell her you are both young and there is so much more to come and there are so many people to meet. Just let her down as easily as possible even though it is going to be very hard. I wish you the best of luck!