Volunteer. Even if you do a shitty job of whatever it is you volunteer to do, it'll tickle the reward center of your brain in such a way that you'll lose interest in self-cutting. You'll feel awkward and self-conscious at first, but that'll go away as soon as you find a routine to slip into.
I could drop some psychobabble BS about humans' ancestry as social creatures, but I'll just say it's more or less guaranteed to work, cheaper than drugs, and doesn't leave embarrassing scars.