"I don't know" used to be my answer to everything in my sessions. My T didn't buy it. I came to realize for the most part he was right.

I discovered as he worked with me that I used "I don't know" mostly in avoidance. I was avoiding emotion--usually fear or potential embarrassment. I was avoiding looking at something that was hard to face. I was avoiding potential disapproval.

My T worked with me to observe what I was thinking (my fear or discomfort) about verbalizing the answers to questions, and I found (eventually) that recognizing why I was being avoidant took the power of that underlying fear away and I could actually answer those questions without total dread.

That doesn't mean that I always know the answers, but I've learned to phrase my "I don't knows" a bit differently. Now I might say, "I really don't understand what you are trying to tell me" or "I'm not sure I understand your question" or "I've never thought about that; I'm not sure how to answer". I've come to realize that if I can't make that kind of pointed response, I'm probably being avoidant. Then I try to slow my thinking down, take a deep breathe, gather my thoughts, and find the courage to verbalize what really is on my mind.

This is a very common problem in therapy, I think because we just feel so exposed. You might try having a talk about this with your T. It might lift some of the blocking you may be experiencing just by directly getting this out in the open.