Maybe better communication. If they had talked more openly about their hopes, dreams, and fears, it could have turned into a real love story. They might have realized they were perfect for each other.
Yes, it could have. Fanfiction often takes existing universes and expands or reimagines them. Harry Potter has so many elements that could potentially be the work of a fan. For example, the magical world with its unique creatures, spells, and Hogwarts as a school. If J.K. Rowling wasn't the original creator but was building on someone else's initial ideas about a magical school and a boy wizard, it would be fanfiction. However, she is the original creator, but the conceptually it has elements that could be seen in a fan - created work.
An unfinished love story often leaves readers with a sense of longing. It could be that the author intended to show the unpredictability of love. Maybe the couple was on the verge of getting together when something sudden, like an illness or a job opportunity in a far -off place, disrupted their relationship.
Perhaps it was a lack of communication. Without proper communication, misunderstandings can easily occur and drive two people apart, even if it could have been a love story.
Authors of Merlin fanfic often do things differently by imagining Merlin in modern settings or giving him a darker personality. Another possibility is changing the relationships between the main characters significantly.
Meet new people. Go out and socialize. You might find someone who can start a real love story with you, instead of dwelling on the one that could have been. New relationships can bring new perspectives and help you heal from the disappointment of the unfulfilled potential of the previous 'could - have - been' love story.