You're definitely not alone with this. I have a PTSD diagnosis, too, and it has taken me a loooooooong time to be able to talk about it in depth with my counselor.

As everyone else has said, you move at the pace that you have to. When you're ready to talk, you'll be able to. Maybe you and your therapist can set a different short term goal to take some of the pressure off you. For example, early in my therapy, I focused on developing friendships and support, and I also talked a lot about my self-critical 'voice.' Talking about how I criticized myself is one of the things that helped me feel safe enough to broach the subject of the trauma.

You're brave to be going to therapy and even more so to be willing to face your PTSD. It will happen when you're ready for it to happen.