((hugs)) losing a T that said they would not be leaving any time soon is really difficult. losing a t at any time is really difficult. It's ok to be hurt, sad, feel let-down, and like it's a huge loss (it is!).
I hope you can be kind to yourself (in the therapeutic sense of the word, not the way I so often use it). Will you have time to say a proper goodbye to your T? If so, I hope you can make use of it, and I hope she can help you transition to someone new... I know though, it really sucks...