Maybe talk to someone that has that mental illness,and ask them about it so it is not clich, like most think the main symptom of ADHD is distraction, but it is not. It is a symptom of something of a bigger symptom. People with ADHD have lacking chemical that gives most people stimulation. So the distraction is because they are looking for stimulation to fulfill that requirement, just like food is for you. ADHD can also micmic other mental illness such as Bi Polar, and OCD, and some get mistakenly diagnosed with Antisocial personally disorder which is NOT true at all, for most of them have plenty of empathy.

if you realty want to be a good writer, it takes research, and first hand experience. They say you should NEVER write about what you do not know, only write about what you DO know.

But most adults with mental illness understand their symptoms better than most doctors, for that is all they have. Like the prevalent myths about schizophrenia. i know people personally with schizophrenia,and they do not fit any of the stereotypes.

ANother thing that is important is most with mental illness, especially those seeking help are more likely to hurt themselves, and than anyone else. Too much media attention is given to mental illness sin the wrong way, and make people with a mental illness continued victims, for they are the easiest to bully,and abuse,, and have the hardest time standing up for themselves, and the majority of people will not either, because of the myths. So please be very careful of not continuing those stereotypes, and myths.

Actually a good movies that i think takes a very sensitive, and comedic approach in telling a story of mental illness is "The Silver Linings Play Book" Awesome movie about mental illness, and finally someone gets it.