Emma -
I think you had every right to feel like you were being interrogated, belittled, and discredited. And it's certainly understandable how she brought back feelings and emotions of a time that was not so happy.
Pamela sounds very much to me like the type who believes that if you're not about to kill yourself, or castrate yourself, you are neither serious nor truly transsexual.
She also reminds me of the type that believe there is no such thing as a different path for each of us - if you are trans, you will do everything within your power yesterday to transition.
Pamela is just one person in the group. No matter where we go...there's always that one person. That one bully. Or that one butthead. It sounds like at the moment, you are the "youngest" of the group (not in age, but in experience), the vulnerable one, the kind she likes to prey on. Sounds like she's damn good at it too.
I think maybe you have allies in other members of the group, and hopefully in the therapist. And of course, as Christie pointed out, you have TGG, too.
-Mike