It doesn't always have to be rigid. People without eyesight say "I am blind," rather than "I have blindness." Under most circumstances, though, I prefer to say I "have," rather than "am." This is because "I have" describes an ongoing condition, while "I am" can be taken as what's going on at the moment. To say I have the chronic illness of depression is one thing, but say "I am depressed," and immediately people are going to ask that question I hate so much. "What are you depressed about?" It's an illness. It doesn't have to be *about* anything. Even on a good day, when I'm not feeling bad, I still have the illness.

In other instances, the "am" is just plain wrong. To say "I'm OCD about washing my hands before eating," for example, is both medically and grammatically incorrect. Washing your hands before eating is merely a good habit, not OCD, and it doesn't make sense to BE a disorder. Even in AA, they say "I am an alcoholic," but they don't say "I am alcoholism."