It was a confusing ending though I suspected that he might not be the princess at all but someone just posing as the princess since FL told him about her all the time.. but as I analyzed the story, turns out, the princess is a witch who does not age.. so it's possible that ML is really the witch and just turned into a man when she found out that her lover has been reincarnated as a woman, hence, the cross-dressing. But it's really confusing. The premise was interesting but the story-telling is not that good.
Watashi no Hime-sama ga Konnani Zetsurin na Wake ga Nai