You didn't say if you are bipolar or not, but if you are yes it is possible for it to spiral down before anyone can predict it. With what you said you had been telling you therapist everything was fine. There was no way for her to know that you would spiral down. Even if you aren't bipolar things can spiral down so fast even you can see it coming on. I think you are a little unrealistic in blaming your therapist for not noticing. Hope you get to feeling better soon.
Gayle