All behavior is communication, but I agree with others that it's best to be more explicit. If it's hard for you to do that in person, then he's just going to have to go wild with abandon and read an email from you.

His approach--prolonged exposure therapy for CSA--is enough to render most people incapable of functioning. So I think your need to skip a session is understandable, but he needs to know why, and then reconsider his approach. Maybe I am wrong, but I have never heard of that kind of approach being used for multiple interpersonal traumas in early childhood. If it's taking a toll on you, I would holler. Loudly.