The funny thing is that the cutters I have known actually were better off than some. At least they had some kind of release.
The vast, vast majority of cutters/self-harmers keep it to themselves, so I'm sure in my life I've known more than two. But most keep it locked up because of people like SaskaBushFire who says "cheer up jerk". And that attitude is common. It's the Dennis Leary school of psychology - "Shut the f'ck up! Next!"
For most, they get over it. I had suicidal tendancies in high school. Knife in bathtub and couldn't do it. Stupidly enough, it was only because I was afraid I'd screw that up and be forced to go to school with scars on my wrist. It's amazing how much things get better once you are out of high school. Or maybe it's simply because the suicidal kids or cutters turn to alcohol and drugs in high school and university and that becomes the all consuming release.
BTW - cutting is far more prevelant in girls than guys. I don't know they don't just shut the f'ck up and get happy.