I think regarding Nate not being able to accept Blair being different is understandable. Not that I like Nate/Blair, because I don't, but since the first season it was explained that they had been together practically there whole lives.
So, Nate has this view of old Blair that he really didn't truly see until they were together again. It happens all the time in real life, when a soldier has been fighting in a foreign country for a while and finally comes home, a lot of times they and their significant other have a had time reconnecting because seeing and being around war changes people.
Not to say that's at all similar to how Blair has changed, but hopefully you get my point.
It makes me happy to see that even though some of you are fans of Blair, you agree that she deserved her comeuppance, which I have personally found rare to see because most Blair fans are whiny 13-year-old girls that have to defend any one who doesn't completely adore and worship Blair like they do.