I think the best way to go about writing mentally ill characters is to talk to someone who actually deals with the disorder on a day-to-day basis. If it's something severe enough that it would be difficult to discuss with an actually sufferer, I'd suggest talking to a medical professional trained to understand that kind of mindset.

That said, anyone who wants to know anything about writing characters with a slew of crazy emotional disorders, I'm your batshit crazy kid for that.