Is a person born with their spouse already destined, or do they have a role in determining that, and what is the supplication that can make them forget this matter?

There is no specific supplication for forgetting someone. However, a Muslim should supplicate to Allah in a manner befitting their situation, asking Him to remove from them what their soul has become attached to but cannot attain, and to replace it with something better. For what transpires in marriage is by the decree of Allah.