A therapist will surely respect your request if you just tell her you're not ready. As others say here, it's your hour and your money, and you can go at things the way you need to, or put something on the shelf awhile. And the therapist is trained to notice if you're scared and don't want to be pushed. Most of all they don't want to push you into fear you can't take, and drive you to terminate or get more serious symptoms.

If it's one of those subjects that keeps bothering you to talk about, something that works for me is to just go at it indirectly by talking about other people's problems with it in general, just to get used to saying the words and hearing that T isn't judgmental or clumsy with it. Bring it on gradually, maybe mixed in with some other talk, instead of jumping into the deep side of the pool.