Multiple Personalities Support Group
Dissociative identity disorder is a diagnosis described as the existence in an individual of two or more distinct identities or personalities, each with its own pattern of perceiving and interacting with the environment. At least two of these personalities are considered to routinely take control of the individual's behavior, and there is also some associated memory loss,...

You are comfortable with your friends and have seen them in many situations. But often in therapy (at least for myself) we present our healthiest, most capable self. Or at least an acceptable crazy us. Animals do this, they dsont want to beseen as vulnerable and picked off.
If you haven't been with them long, it could be that there is one of you who dominates in session and she hasn't seen any other alters. If its possible to go to sessions and concentrate for a while on some other aspect, say stress management, and learn what you can about something asidebfrom the DID maybe at some point your alters will be relaxed and find their ways out.
I think sometimes we feel like we need a stamp of approval from someone that says what we already know. I wonder if you go with the thought that you want to help yourself, and not worry about the diagnosis part (which is really hard) your alters might demand acknowledgment sooner than you expect?
But validate and believe yourself
Willow (formerly C.) Elisa and April.
Because you are not aware and in control of all the different facets of your personality and sometimes (or often) react in unpredictable ways means you are disassociating. The cold hard fact is all those varying fragments were fractured from the real you.
Think of the fractures as small streams and rivers made of your own life force. What is supposed to be pure is filled with all the excess emotional garbage that was dumped on you as a child, all your fears, self-doubts and the things that would have crushed a weaker person's soul. All the things that were said and done to you were a result of them being broken, not you. Maybe this damage was so extensive, it's the reason you fractured, but you can heal and be the person you were meant to be.
Your job is to begin slowly sucking up all that toxic fluid, filter out all the negativity and toss it away where it can't hurt you. Once you realize that every little thing that was done to you was not your fault, was so horrible that it damaged your soul and NO ONE should have ever subjected you to such things, sorting through it will become easy. You are simply sifting through to remove their contamination from your life force. Once one small stream is clean, you integrate a small piece of your life force and grow stronger. Each time you deal with a trauma you grown stronger still.
After you filter enough of these streams and rivers, you heal. Maybe you will always have a protector part to your personality that steps in, but by and large you are integrated. All the pieces that were once broken away and acting independently are now part of the whole. You don't lose anything, you just grow strong by unification.
Continue with your therapy focusing on healing your traumas. Don't keep chasing around trying to prove DID, cause in the general scheme of things it doesn't matter. Let you therapist think whatever she wants. You alone know the truth, and no one can ever take that from you.
I'd sit down and make a list of your six most serious traumatic moments, most pressing issues, biggest fears or whatever you see as holding you back. DO NOT give the therapist your list. Keep it in safely your pocket. Pick the least stressful one, share it with your therapist and let her do her trick. Once you get closure on that issue, bring up your next least stressful one. If you work from least stressful to most stressful, you are in effect cutting the legs off the monster before you attack it. This will make it easier to get your head around it, break it apart and deal effectively with the big one. Plus you will have the strength of knowing you can do it, because of all the smaller successes that came before.
Pretend you're playing super mario brothers and save the big boss for last!