In some ways yes Xander didn't fight enough for Cordy but Cordy made it crystal clear that she didn't want to be back with him she kind of explain to vampire Willow that she liked the thrill of sneaking up and snogging behind people's back that was it. But I think she just couldn't forgive him, if he didn't hang with Willow anymore and stayed away from her then maybe she would of got back with him but because he is always there with her it would of made it hard for Cordy to ever trust him again, if she found out about the dress then maybe she would of got back with him but Xander knows that he will end up hurting her again. Well that's my opinon 🙂, I do love Xander and Cordy together but hey we wouldn't of got Xander/Anya if Cordy stayed with Xander.