That sounds so hurtful. I would react in a very similar way if that happened to me. You've had a ridiculous amount of stress and no way to relieve it healthly, and then the source of your 'relief', your therapist, disappears. I think anyone would be angry, hurt, upset, frustrated at that!
I think it is quite bad form for your therapist to cancel a prearranged appt for a seminar. She had plenty of notice of your appt and she should prioritise prior engagements. If this seminar is so uber important she simply
has to go to it then she should have at least offered another appt or stated that she unfortunately didn't have any more. She should have, at the very least, acknowledged that you may feel disappointed by the change in plans. I don't need to be a therapist to know that if I cancel on someone at the last minute it might piss them off slightly! For me in this situation the key part that stands out is the total lack of acknowledgement. My therapist will say 'this break will be hard for you won't it?' even if she has no intention or way of making it less hard. It doesn't take much effort to acknowledge and think about how someone may feel!
And I don't understand this reasoning that maybe your therapist didn't realise how important it was for you to have therapy....you turn up every session and even if you don't talk endlessly if she's anywhere near human she'd notice that it helps you enough because otherwise you wouldn't bother going! So no I don't believe that reasoning either.
But at the same time I doubt you would go to a therapist continually if you felt she didn't care or wasn't offering you any help. I feel she has made a mistake which hurt you very, very badly....but was a mistake. I don't think she thought what she was doing through and perhaps was simply seeing it from a time-tabling point of view.
I wouldn't be embarrassed at all about your melt down or subsequent contact with her. Nor should you feel guilty for not talking in appts and think you caused this situation to happen. I think you are perfectly within your rights to express how hurt you are, and you did that in the only way you knew how. You are in therapy to deal with feelings like this, if you already had it down then there would be no point in going!
I don't think you've burnt any bridges, I think you finally managed to express yourself!