Sssnoopyyy:
He has ah nian and other ppl/wives around him but XY is the one he loves - so he will always feel empty. Lost her forever
True.
However, readers are told that Ah Nian was becoming more and more like Xiao Yao. Also, she had known him since he was a hostage prince so she knew about the real CX compared to others in the harem.
Readers also know that Ah Nian would have CX all to herself for one month in a year compared to others who barely saw him at all.
CX respects strong willed women like his warrior grandmother, mother and aunt (XY's mother). Ah Nian also fought in the war against Chang Xuan so these are the hints.
Although CX would lose and miss XY forever, he got Ah Nian and Hao Ling in return, a place where he spent his youth and happier times.
I think it's a decent ending for CX.