You're not alone, that happens to me also. It makes me completely freeze and I can't talk and can't make eye contact. We've spent a lot of time talking about that response when I'm not feeling it. To be honest, just talking about it has helped tremendously. We came up with a plan for when it does happen and we just stop whatever we were talking about and focus on grounding. During that time, my therapist just reminds me that we can do grounding as long as I need, which I find helpful. I still frequently don't say what I'm thinking because of it in session, but always make a note of it afterward and often am able to bring it up the following week (or even email or text my therapist ahead of the next session that way I'm not having to say it).