If you feel comfortable and secure enough in your relationship, I think you need to just tell her what happened and about the abuse. It is amazing how compassionate and caring people can be, especially if they love you.
Are you seeing a therapist who might be able to help you with your cutting problem? I can hear you asking for help, maybe your gf could assist you in finding some one to help talk to you and help you.
We are all here to support one another...hang in there and get the treatment you need...I wish you the best....you are a good guy and don't forget that.
With warm regards,