I've seen my therapist on and off for about 6 years, with this last run being about 2 1/2 years. Typically, we have a set scheduled time (as
@chaotic harmony has described). Early last year, however, because of my schedule, we had to flex around a bit. Now, my guy is not terribly organized and he self schedules and I honestly think he works best if everyone has a set time. Within a period of 2 months, he double-booked me with another client; he cancelled twice and finally, he totally spaced our appointment.
I mentioned my distress with the cancellations and the double-booking but then "let them go" (at least superficially) but the missed appointment - whoa. There were awakened abandonment issues, there were awakened attachment issues, there were awakened transference issues...and while he was apologetic and offered me 1/2 off my next session, it all felt too non-nonchalant to me. I was so hurt and so angry and so...
My between therapy communication method of choice is my therapists' answering machine - and after the amount of time we've been together, he is well aware that I process things on his machine until I'm ready to do it in person. Unfortunately, his machine has a time limit of about 2-3 minutes. I must have left him 20 messages walking through every feeling I had...until I reached my decision that his screw up did not negate the benefits I got from seeing him - and I was ready to discuss all of this, first by phone, then in person.
What I'm trying to say with this example is this: your feelings are valid. If he promised you a set time and gave it to someone else (and how you would know that he gave this time to someone else is another issue in my mind) - then he screwed up. If he can't understand that, for clients with attachment issues (and, from what you've written - a history with inconsistent, unhinged therapists), this sort of behavior could be extremely damaging, then he's not as experienced as you (or he) thought he was.
I think what others are trying to point out (and what I'm going to repeat) - what is it that you need to move forward? If this rupture crosses your line, do you need to terminate with him? Or do you think you can work through this? As others have said - what would that look like? Beyond the present "dick move" on his part (and I think it is a dick move) - what else is this triggering? Can you work with this? Can you work with him with this? Do you want to? (I had a therapist that screwed up very badly very early in our work together and decided that while I could work through it with him, I didn't want to spend 6 months on that kind of work at that time). This could be a really good opportunity for you to look at some of this. This is your therapy - your decision.