Sorry to hear this Cinnamon Stick. It is natural to be angry and hurt by this, especially after 12 years, so allow yourself the space to experience whatever feelings emerge for you.
To offer a bit of balance, I do agree with Merecat; we cannot know why she has left the job. Perhaps she was under financial pressure and had to take a full time role, and once you are offered a job, oftentimes you aren't given a lot of choice about how long you have before the starting date. It seems obvious the that she never intended to hurt you. And while you have every right to question her commitment, it seems to me that seeing clients for as long as 12 years does show commitment to you. Not many of us on this board have been lucky enough to have such consistent treatment for so long.
I hope the termination can be a positive one, not least because I think it will make the grieving process easier for you to know things ended well. Also, might it not at least be worth asking T about possible referrals? You don't have to take them, but at least the option would be open and the referrals we be selected just for you, by somebody who knows you very well.
I so sorry you are facing such a loss, and wishing you well for the road ahead.