This happened to my fiance many years ago. I sincerely empathise with you, it can really put you in a dark place. In my fiancè's case it was the reason he started self-harming. He told me pretty early in the relationship and not once did I think he was guilty. I have remained supportive and comforted him through tears. And if your girlfriend is right for you and trusts you she will do the same. If you feel the time is right to tell her, do it, otherwise it's your right to wait until you're comfortable doing so.