I think for some people it is easy to start again because it works, even if it is only temprory. I don't think you should be so hard on yourself. You may have slipped up, but you are just trying to cope with things. Excluding my therapist I was never able to talk to anyone about self harm. I just thought most people wouldn't get it. I hope your friend is supportive and understanding. It sounds like that is what you need right now.