Hi,
I have social anxiety and some agoraphobic tendencies too, probably from the PTSD.

Perhaps it would help to start with something a little less...direct...than talking about the events that caused your trauma. Instead, maybe start talking about or writing down things you feel when thinking about those events. Then, dig a little deeper and (when you feel comfortable--work up to it slowly if you need to) consider the things you felt relating to the traumatic events back when they were occurring. It might be an easier transition from talking about the feelings to talking about the events than just jumping right in and talking about the events. And it may take quite a while before you're ready to talk about the events...but if you move towards it little by little, maybe it will help.

Have you discussed with your therapist your inability to verbalize the trauma? Perhaps he/she might have some better ideas for ways to do so that aren't emotionally avoidant or retraumatizing.