Zay actually has gone through many timelines while trying to save Aseph but every time she would fail and sacrifice herself so Premestia would bring her back to try and change her fate but she always chose Aseph
I can totally understand her despair here I mean she spent a year before entering the story agonising over his situation, and when she was finally given a chance to do something about it she is given a fated partner who is potentially not him and the body she possesses is the daughter of his enemy leaving her feeling like she’s doomed before she even began. Yet it can’t change her undeniably desire to save him