Much about the causes and etiology of mental illness remains unknown, so there's no real data to answer your question as to whether any type of mental illness is "terminal." For some people with some disorders, it is likely they will live with a form of the disorder for their entire lives. With treatment (just like in medicine), that "living with" can still result in a good life.

Our understanding of mental illness is still in its infancy in many ways. It's a good question, but one where science doesn't yet have the answer to.