I haven't had a chance to read the book recommended in the last post's comments yet. However, I did skim some online reviews and let me say, that in itself lifted a tremendous burden!
Now when I hear people 110% confident about things where I'm sure I'm wrong, I just imagine that the certainty part of their brain is larger than the certainty part of my brain. It's just anatomy and has nothing to do with fact. Usually I just let it go ('cause I try not to be a right fighter if it doesn't really matter), but in the times where it bugs me, I feel much better about voicing my opinion and standing my ground.
And whether or not it is true, I'm going to apply the brain elasticity theory to the certainty part of my brain and see if I can grow it over time.
Wish me luck!