Emma was your best friend but one day she had to move out.

You tried to talk to her even if it was long distance but one day you ghosted her.

She had a hard time forgetting you, until one day she came back.

There's this trope of the childhood friend doing something bad or being a jerk when you see her again, so why not a plot where you're the jerk?

I would write about the backstory but it's pretty much what you expect. Her character is nerdy and depressed and you can choose the reason for why ghosting her.