I think your email is great. She needs to hear it. Regardless of what's going on for her she should at least be aware of the effect it's having on you and take steps to deal with that, or, if she can't, decide whether she's really in a position to see clients right now. You're her client and she has a professional duty to not make things worse for you if she can avoid it. I'm so sorry that your latest session was cancelled again and I just hope she responds to your email. She needs to communicate with you better, in my opinion.