I think I can see both sides of the coin here. As a professional who meets with clients in largely the same capacity as a therapist, I am definitely guilty of overbooking myself and letting my clients down. On the other hand, I have also been let down by therapists cancelling sessions and discharging me from their services.
My personal experience with letting my clients down stems not from indifference or a lack of compassion, but from feeling a strong need to take care of everyone's needs and keep them happy, which becomes increasingly difficult the larger your client base becomes. I might consider the possibility that your therapist has a large caseload of other clients and has a tendency to underestimate the time he must spend on any given task. I would suggest acknowledging that you are sensitive to the fact that you are not his only client, and that you know he has a family and a life outside of his practice. However, it's not at all unreasonable to ask that he only schedule you at times where he does not have any sort of time-consuming task either before or after your appointment (but mostly before), and that if there is any possibility of his earlier task running over, you would rather he just schedule you at a different time. I would also suggest that you make sure he understands that it would be easier on you if he just doesn't schedule you on a specific day than it would be for him to schedule you and then cancel or call late.
In regard to your ongoing feelings of helplessness and hopelessness (my words), have you considered voluntary placement in a local behavioral health (I hate that term) center just long enough to assist with stabilization? I have done so on more than one occasion, and although it is a little scary at first, it has turned out to be a positive experience each time. I think a large part of that is having several therapists, psychiatrists, nurses, etc. checking on you at least a couple times an hour. Another benefit is meeting and talking to people who are in very similar situations. There are a few places here and there that have in-patient trauma programs, which is immensely helpful.
I would also advocate for seeing an additional therapist. I have seen two therapists for almost a year, and it has been a great help. You don't necessarily need to go into your trauma with your new therapist - they can simply help with day-to-day coping skills, which will arguably also assist your trauma processing in the long run. Even a DBT group could be very beneficial. It just sounds like right now, you're putting all of your eggs in one basket in terms of your dependence on your therapist, which seems unfair to both you and him, and likely to lead to disappointment for one or both of you.
Anyway, I just thought I'd share a few suggestions from my own experience. I by no means have it all "figured out," but I have found these particular things to be effective.
I hope you're feeling better soon! :)