What is the reason for my fiancée's distress, the heaviness of her heart, and her aversion to me, after she was once attached to me, and what should I do about it?

If there is no ordinary reason for the fiancée's aversion, then the reason might be extraordinary matters such as magic (sihr) or the evil eye ('ayn). In such a case, ruqyah (Islamic exorcism/healing) is recommended. It is better for the fiancée to perform ruqyah on herself, based on the Prophet's (peace be upon him) description of the seventy thousand who will enter Paradise without reckoning: "They are those who do not ask for ruqyah." If she needs to seek ruqyah from someone else, she should seek out people of righteousness and avoid sorcerers. If the aversion disappears, then praise be to Allah. If the situation persists, it is better to dissolve the engagement, as it is easier than divorce after marriage. Dissolving the engagement is permissible when there is a need, and disliked without a need. After dissolving it, seek someone else and ask Allah to facilitate a righteous wife for you, for "the world is a provision, and the best provision of the world is a righteous woman." It must be noted that the fiancée is considered a stranger to her suitor until the legitimate marriage contract is concluded, so the Islamic guidelines between them must be observed.