Depends on my mood and state of mind at the time.
Usually I will say "I AM" but, if I'm worried about being judged for it I may say "I HAVE".
I recently was informed of a diagnosis that had been kept from me for whatever reason. For a long time I was sure that I was Bipolar as my previous psychiatrist pointed out once as well as schizophrenic.
The diagnosis revealed to me was that I am Schizoaffective which from what I've read sounds an awful lot like Schizophrenia and Bipolar Dissorder combined with additional symptoms so, if my previous perceptions were incorrect, I wasn't far off of the mark.
Still though, I can't rule out that there is a possibility of having one or both of the previous two as well since people can and sometimes do have more than one similar condition.
I prefer "I AM" though because to me that is part of facing reality and being honest with one's self.