This has happened to me before in session, so I know it’s really difficult. A couple times, ex-T did a relaxation exercise with me. Once, ex-MC had me and H walk outside with him, and the fresh air and not feeling watched (because we were walking) helped. So maybe something like that could help? I would definitely tell your T what you were feeling. I’ve also found with current T that if notices I’m anxious and acknowledges it, that can help, because then I feel I don’t have to hide it. Or if I mention it to him first (though he can generally tell).