You should tell her right away and earn back the trust. Better late than never. Better soon than delayed. If she were to find out way later, she may feel like you have not been honest with her all along, and it would be harder to earn back her trust.

If you do not want to talk about your ex t, you can simply say,

"I didn't have a good experience with her and I do not feel ready to discuss her yet. But when I feel ready; I will."

If your current therapist pushes for information after that, that would not be a good sign.

Next time you see a new therapist, you should tell her the truth, and if it is the subject that you are not ready to face yet, simply let her know of that fact.