I personally feel like you should cut the T slack for one mistake. I think mine double-booked me once and I had to change times (she called about that though--was before I got there), though that was after I'd been seeing her for a while. I understand based on your past experiences that you don't want to--but it's good to give second chances, especially if she was apologetic about it. And both my T and marriage counselor have had to cancel last minute due to illness or family emergency, and while that was difficult in some cases (I really needed that appointment a couple times), I understood. Then again, I'm a pretty forgiving person, and hope that others will be equally forgiving of me. I'm much harder on myself than I am on other people, partly due to how I was brought up.

Sounds like you have your mind made up--I'd definitely discuss this with your next T. Good luck!