How should one deal with a fiancée who tells her fiancé that she is attracted to someone else, despite their impending marriage, and what is the Islamic guidance in such a case?

The human soul is inherently volatile, and therefore this reality must be understood to deal with it and overcome problems. A person cannot repel whisperings (waswas) and changes in mood, and it is natural for human relationships to experience periods of coolness. Great souls are those that maintain their equilibrium, but it is not realistic to expect this from everyone. One should not interpret a partner's or friend's turning away as a change in the relationship; rather, one should preserve feelings, forgive, and not demand that people be as we desire in all circumstances, because a person himself does not possess that ability.

Your fiancée wearing the hijab without consulting you is evidence of her prioritizing obedience to Allah, and her candidness with you about her feelings towards an old friend is evidence of her trustworthiness and honesty. These positive qualities should encourage you and hasten your marriage to her, keeping in mind that a long engagement period and her young age can explain these fluctuations. You must act wisely and not let doubt and hesitation control you, and strive to influence her with your good character, for hastening the marriage is a solution to many worries.