I haven't gotten there yet, but here's what I usually do in that kind of situation. I pause in the parking lot for a moment and remind myself that one of the good things about being an adult is that I can tell someone to F_ off and walk out of the room any time I want to. Maybe not the most mature approach, and I've never actually done it. It DOES help me to remind myself that I can be in control of the situation and that helps.
The first time I ever talked about any of this stuff with anyone, he had somewhat figured it out and worked real hard to get me to talk about it. When I finally did (and, in the end, I couldn't talk much, just answered "yes" & :"no" questions.... When we went our separate ways that night, I was SURE that the world would come to an end by morning. I was sure he's see me differently and hate me or avoid me or think I was "damaged goods" or something similar. That's not what happened at all. The world was a different place the next day, but in a good way. If your therapist is good, that's where you're going to end up too, It's like jumping in to a pool of cold water in the summer. You dread it on the way in, but once you get there it's not so bad.
Good luck! I'll be thinking of you!!