Telling your parents has got to be one of the hardest parts. I did it recently myself (well, my mother, anyway... my father is a rightwing neo-nazi guy and I don't care to know of his opinion yet).
The therapist should serve as a good mediator, and keep the discussion productive and on track. They are unlikely to let it degenerate into a "flying off the handle shootout" without stepping in.
Perhaps it would help to write down some of the things you would like to say beforehand, as it's likely you are going to be given a chance to speak up uninterrupted in a mediated environment.
I know it's hard, but just try to tell them how *you* feel. Be honest. Prepare statements beforehand if need be.
They will have a lot of questions and concerns. It'd be adventagous to try to predict these and work on your answers.
Parents don't *try* to freak out - it just happens. It's a lot of new information to absorb.
They've just found out someone closest to them, someone they gave birth to, is not who they thought they were - it's a hell of a shock (or so my mother said). They have an image of you, and a future for you all planned in their head, and it's all about to dissappear in a puff of logic.
Above all, they're probably afraid for you - they think you're trading a normal life for the life of a "freak", and it's up to you to tell them the truth - that that isn't the case.
Don't expect them to understand 100% of everything, or accept it all right away. You've had much longer to get used to the information.
The most important thing is that everyone remains as calm as possible, and keeps talking. Keep the communication flowing. Starting to talk about it is the hardest part. Breathe, swallow once, look away and BLURT IT OUT!
Once you're going, it's easier to keep talking.
Good luck!
