You already know the story. You've told it to yourself enough times that the middle has worn smooth — the version where you're not the one who left, or the one who didn't say it in time, or the one who was too young or too scared or just a little too late. You know how it starts. You know the year, maybe the exact street.
What you don't let yourself write is the end. Not because you don't know it — you do, you've known it since the day it stopped being a maybe — but because the last line is where the story stops being a story and starts being true. As long as it's unfinished, it's still a little bit alive. You can still, in some back room of yourself, be writing it.
So write the last line. Not the one that sounds good. The one that's actually true — the one you'd delete if anyone could see it. It might be a name. It might be 'I hope you're happy, even without me.' It might just be a date.
Write it. Then close the notebook. You don't owe it a next page.