Tell yourself you can only do it on certain days. Don't cut down too much to start. So say for instance you are not allowed to do it on Tuesdays, Thursdays and Saturdays.
Keep cutting it down very slowly and you'll be fine. Give yourself a goal to work towards, perhaps so pampering?
Good luck. I stopped biting my nails by saying I could only bite certain ones and not others and have more or less stopped altogether.