It's a tough decision to reveal that kind of stuff.
For myself, I waited for quite a while before I told the person. I didn't want my casual dating relationships to know... I have some visible scars though, so at least that needed to be explained when the person asked.
If you trust your gf and you think she is a safe person, you can start to tell her little things... It might be a bit overwhelming (to both of you) if you spill everyhting at once, but you can try testing the waters with her my telling her some more vulnerable things and see how she reacts.
Just go with your gut and what you are comfortable with. While I would like to assure you that your gf would be fine with it, experience has taught me that different people have different abilities to understand that kind of stuff.
Hope she responds well if/when you decide to tell her.
(fwiw, my wife kinda freaked a bit when I told her, but that lasted a few seconds. She moved quickly into anger at it having happened... I was totally scared telling her, but she's been an amazing support. Hope you have a similarly good response from your gf).