Abstinence & Celibacy Support Group
The pressures of sex from peers, the media and culture can be overwhelming, but you are in total control of your body and celibacy is a choice that many are making. Find sanctuary with people who are choosing to abstain without fear of judgment.
Simple honest words are more kind than false gentleness. Don't shout. Expect a tear or two but to drag it out would be unkind. Just cut her loose.
The other piece of advice I have is to take the blame. If something that she is doing makes you want to break up with her, thats your fault. You can't change another person, you can only change how you react to them. If the reason that you're breaking up is something that you can't or won't change your reaction to, tell her that.
So, a blueprint would be to say something like, "You're a terrific person and you are so close to what I want in a partner. But, there are just the times that you _______. I just don't react well when people do that. It's my fault that I'm not more open about ______. I hope that we can continue to be friends."
If it really is just a lack of chemistry, just say that because, chances are, she feels it too.