I agree with AllHeart. There must be something serious behind this but it is no excuse to keep doing it even if she apologizes boldly. Did you ask her why she was not there? I think you deserve an explanation at least and I would tell her this.

My first therapist did the opposite, forgot my appointment cancellations, three times. He would call me on the phone during the appointment time asking where I was and not even apologizing but telling me that I should remind him the day before. WTF? I did not make a fuss about it back then but regretted later as he had a tendency to be sloppy and then defensive in several ways and even offended when I said it was unprofessional. You T does not sound ignorant and defensive like that but still... it is unreliable.