I think you should tell her. Cutting is a serious problem, and it is extremely hard to recover by yourself. :( I know it might be scary for you to tell her, but she might appreciate you telling her instead of her finding out by herself. Then, instead of sending you to a mental hospital she might be more open to getting you individual therapy, which can help you a lot :)
I will judge you if you have more tapers than plugs in your body jewelry collection.