I'm a bit of a chicken, I'd "blame" it on what my husband says, say he hasn't seen any progress and you trust his judgement so you're going to try with another person; not her/T's fault, probably you, yada yada :-) I'd tell her all this at the beginning of the session and then I'd ask if this session was enough or if she wanted you to come back once more, etc.

I'd just keep repeating how I maybe don't feel like I'm working hard enough, making a connection, etc., I would try to keep it away from discussing much about the T and her methods, even though that might be helpful to her to know but that isn't your problem, you're just trying not to rock the boat. Don't discuss having already found someone else or why you like them, etc. Just be your normal self, a little vague, and obviously wanting out of there, LOL.