Ugh. I'm sorry she wrote such a disappointing reply. As I see it, there are two parts of it to sort through with her:

1) Your feelings about her being gone and waiting for the reply you expected from her (based on her promise) and then not getting it.

2) Whether or not she intended to send you the usual email after a long break, prior to your appt, or whether she had intended a special "I'm back" email as a way to ease your fears and concern about her absence/travels.

The answer to #2 is going to worsen or ease your feelings regarding #1. If she never intended to do more than the usual email after a long break, then this was a miscommunication and not out right forgetting. If, on the other hand, she had intended to do what you thought, then she did forget and that would complicate your feelings.

And, as I've dabbled (and gotten lost) in unaluna speak for some time, I think what she's saying is that after you get the facts sorted out, the rest of the conversation is about coming to terms with all the ways other people disappoint us, because they will. I would hate to think that your therapist did that on purpose (and from what you've shared about her, I don't think she did it as a lesson).