Ok I'm just back to say that her advice in her Q&A on how to handle a breakup, that one day you will meet someone "strong enough and clever enough" to love you just totally underlines the point I made earlier, that she doesn't actually reflect on how she ends up in relationship dynamics with men who blindside her with bad behaviour, according to her own descriptions. Oliver just wasn't "strong and clever" enough! That's the issue, we're done here.
Also to agree with what several of you have said, I think Carrie was probably ignoring signs of trouble. I have also definitely been the person who "completely blindsided" someone else. It was a friend in my case. I spent so much time and energy trying to communicate with this friend why her behaviour was really bothering me, and it didn't matter. It just continued. Then when I finally walked away it was "so surprising" and I just unexpectedly dropped our friendship.
It was definitely also a case of me thinking our constant arguments and tension were abnormal and really exhausting, while she thought that was a normal part of friendship. I could definitely see something like that happening with Oliver and Carrie; sometimes the one who had no idea the relationship was going bad is just the one who has a very high tolerance for toxicity.