I say I don't know a lot as well. One of the things that has helped me is from the beginning my T would ask me after I said that was, "Do you not know or can you not say?". I would answer can't say or don't know according to what the answer really was. If I said, "Can't say", she'd say okay. She wouldn't push it or question why I couldn't answer it and her just accepting that I couldn't say was enough for over the next while for me to start saying a little bit more. I can't say because I'm embarrassed, or I can't say because....

Not sure if that makes sense but that's my history with the "I don't know".