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I think it depends. I've seen people quit drugs and never think twice about it. The withdrawals sucked, but some people have the willpower and determination to get through it just fine. Of course, it also depends on how much of the drug, what kinds of drugs, and how they quit.
With cutting, again it depends. Some people feel so shitty when they don't that they can't fight the urge. For most people it's just a phase, but some people really do have such horrible lives that it's understandable why they'd do it.