I would assume that the main characters are the two friends. One is the protagonist, the person whose perspective we are seeing the story from, and the other is the friend who is no longer in their life as closely. There might also be some secondary characters who play a role in the events that led to the friendship changing, like other mutual friends or family members who influenced the situation.
The main theme could be about lost friendship. It might explore the reasons why a friend has drifted away, like changes in life circumstances, differences in values, or simply growing apart over time.
I'm not sure without reading the story, but it could be a protagonist who is haunted by the 'one that got away' and perhaps a secondary character who was part of the situation that led to the loss.
The main theme could be about lost opportunities. It might revolve around a character who misses out on something or someone important, like a relationship or a chance at a better life.
The sense of loneliness can be tough. You might have shared a lot of things with this friend, like going to the same places or having inside jokes. When they move away, those things might not be the same. There could also be a fear of growing apart. You worry that the distance will make you less close over time. And if you don't communicate well, misunderstandings can easily happen.
You can start by expressing your feelings to your friend. Share your sadness and also your excitement for their new journey. Stay in touch through calls, texts or emails.