This is something all writers know about. The middle section of your first draft will sag like mad for a while. You can’t properly write it until you have the ending written.
Something similar happens is editing. The drafting process is this mad dash to get the story out, and the ending (publishing) is a mad dash to get people to read.
But the middle is where you have to work, hard. Smoothing out every sentence, cleaning up the language. Fighting with the betas who insist on switching up a name and pronoun here, even though last week they wanted it the other way around!
Sorry that last one was a touch too autobiographical.
And it gets tiring and frustrating, and you wonder why you even bother with this? But then you open scrivener and fix it all up.
Because the answer is, you’d do this anyway. I wrote stories in my head for years before I committed one to ‘paper.’ If I gave up on publishing tomorrow, I’d still write those stories in my head.
Although, on the plus side, I wouldn’t have to mentally argue over my use of ‘snorted a laugh’ over ‘snorted.’