I hope today goes well for you! I don't know if this will help, but it seems like you've already come up in your mind with the worst-case scenarios of how today could go. Even if you do dissociate, it is not a wasted session - you are there with your T, you're doing your best to talk about a very difficult subject, and your T is supporting you. So no matter what happens, you are taking steps to take care of yourself and better yourself - know that, feel that, own that.