I can definitely relate to that. I say "I don't know" a lot too and I know my therapist gets quite frustrated because of it. I know that it makes me come across as defensive and a bit passive so last time I saw him I finally tried to explain why I say "I don't know" that often and I hope he understood what I meant.

For me it's very difficult to translate my thoughts and/or feelings into something I can express in words that my therapist will understand. When he asks me questions my head is very often filled with thoughts that don't know how to translate into words. Sometimes there are so many thoughts in there that my brain just "freezes" and shuts down. The fact that I then tell him that "I don't know" does often not mean that I actually don't know the answer to the question (though that of course happens too). It just means that I can't communicate well enough or that I partly know the answer but that I can't express it because I fear that he won't understand me if I don't express exactly what I feel (the whole answer, not just a part of it).

It's a bit difficult to explain but do you know what I mean?