"The agonizing self-doubt is always there, of course, and I have to remember that this novel is like all others, a continual effort to surmount it and spur myself on like a rider through a frustrating thicket."--May Sarton, At Seventy
I don't know why stories (and poems, and essays, and books) rarely manifest themselves fully sculpted, but in my experience they don't. In my experience they prefer to be hacked from the ground and polished painstakingly over time.
I'm working on a project right now where the ending is performing its usual hide-and-seek routine. I know the plot has essentially concluded, but the story needs a final something ... something ... what? I know the ending must be around here somewhere. I can almost feel it!
The only difference between my process now and many years ago is that now I don't try to force it. I inch forward through the thicket Sarton describes, untangling the briers from my hair and clothes, looking for the path forward. I may only advance a step at a time; I may have to backtrack. But slow progress is still progress.
In hindsight, it will seem as if the finished story was always there; the ending was inevitable. Right now my characters stumble around, following my try-this and try-that cues, and the final scene seeps forward, a few lines at a time.