Your letter is great! The only thing is, perhaps you could put in a really explicit description of what happened, "the facts". You called, found out your appt had been cancelled, but T didn't know why (and didn't do anything about it). Then T rescheduled you, but you found out that was for a day when she wasn't in.

(I don't remember whether this is exactly what happened, it's just an example.)

It's just best to at least be on the same page about exactly which incidents are making you feel this way, to avoid any extra layers of confusion... it's going to be hard enough to talk about your feelings around it without also having to remind her and re-explain what actually happened.