Piper, you are being brave to be in therapy. I think you know that trying to keep big secrets from T will only prolong your healing time. Of course, it takes time to build trust and if that is the true issue, then take the time you need. I think most Ts accept dissociation. Is it abandonment in general that you fear? Many, many of us here are very sensitive to abandonment because we were either told or made to believe we weren't worthy of being care for. We were hurt instead. If that is true for you, could you start by talking about acceptance and fears around that in a general way? You may find the reassurance you seek and thus the safety to show more of yourself to her.
I wish you to find safety and trust so that you can move forward. if safe