This certainly is not an issue of an emergency or anything like that. Rather, it really seems part of the therapy progress and thus it at least according to my view seems completely fair that you would pay for that session, regardless of whether you decide to go or not.

However, I also think it would make perfect sense to raise this issue with the T and pressure him a bit to see how he handles the situation. Because my guess is that's what you really want - to see how he would handle it. I doubt you would be happy if he would say that you're cancelling would be totally fine because he would just see someone else (a potentially new client) at your dedicated time slot.