In the movies, writers are usually shown pounding out words by the baleful, rarely pausing and never stopping. Sentences pour forth in a cascade, as a cataract on a river, and that first effort is exactly what goes to press.
In truth, writers work very differently. Take Drayton, for example. I have a draft of the whole novel now, but am not even nearly finished. Instead, I am taking each chapter as it stands and rewriting it from the top. Right now, I am at work on Chapter 11, and it has changed almost completely. I've hardly relied on the previous version, but have it printed out as a guide to use as I go along. Most of the chapter so far has gone well. I have probably about 1800 words so far and am very pleased.
However, I spent the last week reworking three paragraphs and slowly adding a fourth. I go back, every day, and make changes and then change the changes. I think I'm almost finished, but only time will tell.
And this how writers really work. You write and then you revise and rewrite. You change and you change and you change, and then you get an agent who wants changes and an editor who wants even more. And you work and you revise and polish and never actually get it to the place where you really want it to be.
But that's the process.