I have bipolar disorder, and I've self-harmed (some minor cutting, but mostly burning/branding my arms) in the past. My experiences are slightly different from the stereotypical ones in that it's never been habitual or even deliberate for me. It's like I go into a trance and just find myself doing it - I've never actually want to. Then I snap out of it and completely freak out when I realise what I've done. It's absolutely terrifying and deeply upsetting. I'm immediately filled with an overwhelming sense of shame and go to great lengths to keep the marks hidden (the whole trance thing means I don't have the forethought to do it somewhere like my thighs or upper arms). Fortunately I haven't been left with any obvious scars, and the few I do have aren't really "suspicious."
That said, the fact that it's such a rare occurrence means that I can use it as an indicator that I really need to do something about my illness. The first time I did it was when I was first running into serious problems with the bipolar (though it wasn't diagnosed until years later), and no one around me was taking me seriously. I was completely overcome with anger and frustration and ended up burning myself. However, that made me realise that I needed to talk to someone, so I started to open up about the problems I was having to a trusted teacher. The last time I did it, last summer, the bipolar meds I was on at the time just weren't helping at all and the side effects were awful, but I denied there was a problem for months (I'm sure lots of you with experience of mental illness will know the feeling). When I found myself cutting, it was the kick up the backside I needed to go to the doctor, get referred to a psychiatrist again and change the medication.
I was also bulimic as a teenager, and I think that plays into the same self-destructive mindset, which can also be a feature of bipolar.
For the record, I'm 22, female and bisexual, so I guess at one point I fitted in with the teenage girl stereotype. I was all too aware of that at the time and felt so disappointed in myself for becoming stereotypical, what with the eating disorder and everything as well. Stupid way to think about mental illness, but I was young and uneducated about this stuff back then.