I don't know what stage you're at in your relationship, but I've been through where you are and I have some advice. If I were you, I would tell your gf something basic for now, like "I had some really bad experiences with sex, so it can be hard on me emotionally." And let some time go by so she can get used to that. I've found that sometimes even a very understanding person can get freaked out if you tell them too much too fast. You want to let her in, but I think it's important to do it a bit slowly, so that your relationship grows and she feels she can handle it. Anyway, this helps her find a role to help you; if you tell her that during sex you need extra emotional support, that tells her just what she needs to do. Once she gets used to that, it will be easier for her to listen to the whole story without getting freaked out, and she will value you for the person you are... Just my thoughts, because I've tried everything, and my story is awful too....
good luck, friend,