"I have" for sure. Think about how "I am" works for other illnesses:

"I am cancer"
"I am diabetes"
"I am depression"

It sounds quite silly, doesn't it? But for some reason with Bipolar Disorder, it is quite popular to say "I am Bipolar". Same goes for "I am OCD". Perhaps because some people feel like the illness defines them as a person, which is rather unfortunate. We are so much more than our diagnoses.