I use "have". Using "am" feels too self-defining and I am so much more than a diagnosis. All of us are.

Also, saying "I AM Bipolar" reminds me of all of the ignorant people who say "oh my gosh your so bipolar!" When someone is in a bad mood or gets frustrated over something. It's also thrown around as a bullying word. "Don't talk to her, she's crazy and bipolar".
Gag.

-Ally