I think it is unlikely that a therapist who specialized in DID would think a person was DID if she wasn't. Being a specialist doesn't mean you have a hammer and nail everything in sight, but that you can discern between finer distinctions than a non-specialist.

But if you spend all your time related to therapy questioning everything she says about DID and your diagnosis, then you don't have to deal with anything else. It might be worth it to explore why you throw up this kind of roadblock to therapy and whether there is any there there.