Some Ts just don't believe in DID, period. I've been to one of them - it sounds like you have now, too. If you do have DID seeing a T who doesn't believe in it isn't going to help you at best, and will most likely be very damaging for your system.

Keep in mind that her disbelief in your DID is not about you at all. It is about her training. I know it feels so very invalidating to be told something like that, but remember she is only working with what she knows, and she simply doesn't know enough to help you. I saw a T who didn't believe in DID for a few months a few years ago, and it just wasn't worth it. I felt like every time we switched (it wasn't often as we knew it was 'unacceptable' there, but sometimes it just couldn't be helped) she thought we were faking it. It was incredibly invalidating, and a total waste of time. I gave her a lot of information and research about DID and DID therapy, and she didn't even bother to read it. In her mind it didn't exist and that was that.

In a way it's like a Christian who is having spiritual issues trying to seek help from an atheist. It just doesn't work. I don't have much faith that this T will ever be able to adequately support you. It's okay to chalk this up to experience and find someone who understands and knows how to work with people with your particular issues.