From the little time (just a few months) that I was with the girl I was with, I was never entirely sure. She read a pros and cons list I had made about her to strengthen myself following first break up, and when she saw it she fell apart emotionally - told me that everything I had checked as negatives were true. I harbored a lot of guilt over her seeing that list, but as the relationship continued, the situation never genuinely improved. The negatives I had listed had mentioned stuff like "lacks empathy", "shallow", and "acts trashy around other men" etc, etc.
I think what happens is that they protect themselves by denial - they try to turn things around and tell you that you're the one with the problem - that you're insecure. I'm a strong willed person, but weathering the arguments started breaking my confidence down. If I showed weakness, I got slammed - the power play was awful. And all the while, she was building up alliances with other people against me. These people were often other men that were tripping over their own tongues and would validate anything she said because she left them with the impression that they had a shot with her. That and she ommitted details that would make her look bad.
I think this form of protection winds up perpetuating things and having them stay in denial, though deep down, they must know something is wrong.