I think there's two meanings. 1) Dan did love Serena a lot in the beginning but fell out of love when she kept choosing other guys over him. That made it easier for him to write the truth. 2) he never did love the real her and was just obsessed with her. This caused him to resent her because he was never good enough, even though that's not her fault. The book is therefore payback to make her seem worse and himself feel better
Dan first fell in love with the idea of Serena . When they started dating in season one; he actually got to know her as a person and realize that she was a flawed person , instead of this dream girl that he met at a party 2 years earlier. Dan loved Serena, but he now sees her in completely different light than when he was a kid.
Nope. He kinda admired the upper east side and Blair is a perfect representation of that. But Blair detested Dan in the beginning (since he was from Brooklyn) and was unavailable later, so he never really had a proper chance with her. Hence had to settle for the next best thing- Serena.