How can one determine whether their desire not to complete the marriage, and their feelings towards their former fiancée, are whispers from Satan or genuine feelings? Is abandoning a fiancée without a valid reason forbidden, especially given her strong attachment to him, and the fact that he has performed Istikhara but found no solution? When should the Istikhara prayer be performed, and is it permissible to leave a fiancée solely due to a psychological desire, and does sin result from that?

The devil's intervention is not necessarily involved in your attachment to the first girl and your comparison of her to your fiancée; rather, it could be a natural attachment. Try to forget about the first girl. If your fiancée is religious and of good character, then be keen to marry her, for a religious woman is the best provision in this world. She is affectionate, fertile, and kind to her husband. If he looks at her, she pleases him; if he commands her, she obeys him; and if he is absent from her, she protects him. If you are compelled to break off the engagement, you will not be sinful for doing so, even if there is no specific reason, but do not rush. Seek guidance from Allah (istikharah) and repeat it, for it is not a condition that you see a specific sign afterwards; rather, whatever Allah decrees for you after seeking His guidance is what is good.