Dear T,
Thanks for being so accommodating and fitting me in for an extra session at the end of next week, since you'll be away the week after that. And I appreciate your confirming that you'd still see me at my regular appointment without my having to ask--I think you really are figuring me out!

Also, note that I avoided any apologies for contacting you again or any comments fearing I'd be asking too much or taking away time away from you or your other clients. I'm trying to just take you at your word that I both haven't crossed any boundaries (so the scheduling text is fine) and that I don't need to "take care of" you (or your other clients). That's at least a tiny bit of progress, right?
LT