How can one deal with a relationship that involved phone sex and the exchange of nude photos, given the existence of a marriage promise, feelings of guilt, and distance from God? Should this relationship be abandoned, or should the marriage promise be fulfilled?

The relationship with the girl must be immediately severed, and you must repent to Allah for the forbidden acts that have occurred. Repentance involves abandoning the sin, regretting it, and resolving not to return to it. Marriage to the girl is not obligatory, and if the girl repents, there is no impediment to marrying her. The important thing is repentance and returning to Allah, striving in His obedience and supplicating to Him, while also ensuring good companionship, listening to beneficial admonitions, and frequently engaging in remembrance (dhikr) and supplication (du'a).