Mis(sed)diagnosis used to happen a lot more than it does now I think, ES. As more and more research is done therapists are getting more comfortable with the concept of trauma-based dissociation. That said, if you ever find yourself in the company of a therapist who leans towards a diagnosis of psychosis and is unwilling to explore trauma and dissociation with you, vote with your feet and walk the other way.
I have had to do it. Around ten years ago a therapist I was seeing kept talking about schizophrenia and DID as though they were one and the same. She was unaware of her own ignorance. Stuff all that crap about listening to treatment providers and letting them tell you what is going on in you. If your treatment provider doesn't know what they are talking about don't listen to them!