I used to face this is my earlier years of therapy. Lately if this happens I sort of "talk around" the issue, saying a little bit, but not much, not even so much that T knows or can figure out what I'm talking about, and all the time repeating, "I know I'm just talking about talking about this issue, I am not ready to talk about the issue itself." After I do this for a while (5-10 minutes, or a few weeks), I usually feel comfortable enough to actually say what the issue is. It helps that my T knows me very well and knows when to say, gently, "Why don't you just say it?" If I feel ready, I do. If not, I might get angry, and tell her that, too.