Same here. I'm lucky that my therapist sees through it... on my like second session she said "Usually when someone says 'I don't know' it just means they don't want to talk about it" and so when I say "I don't know" she doesn't let it slide, she lets me sit there until I've decided what I can say. Sometimes I truly don't know and she understands that too.
It makes therapy very difficult but then again if therapy was easy it wouldn't be very helpful.