The email was to your therapist, not the person who diagnosed you, right? Is there a way to get an appointment with the person who diagnosed you to get some more understanding about alters from her since she has more knowledge about dissociative disorders? To me, talking about alters or parts is a way of talking about my internal experience so that therapists and I could discuss some things that aren't usually talked about in ordinary life. Since your internal experience is unquestionably something that you experience, it's real in that way, whether you use the language of parts or something else.