How can I deal with my conflicting feelings toward my current fiancé, given my continuous comparisons to a previous unlawful relationship, and my feeling of unease despite having performed Salatul-Istikharah?

One must hasten to repent to Allah Almighty from the sinful relationship by abandoning the sin, regretting it, and resolving never to return to it, while ensuring the heart does not dwell on it. This is because a weak resolve and a heart that still yearns for the sin are indicative of an incomplete repentance. One must thank Allah for the righteous spouse, forget the previous relationship, and begin a new life founded on obedience to Allah.