And then I remember that to get to that final form I'd written and re-written and edited like mad. Then it went out to the friends who read for me (that's the stage I'm at now). Then it got re-written again. Then my agent saw it, and I incorporated her feedback (it's not usually much, as she hasn't come the editorial route). Then my editor. Then I re-wrote it. Then I had some more feedback from my editor. Then I did some more editing. Then a copy editor saw it, then a proof reader.
So, I shouldn't be comparing the stage I'm at now with the finished product. And nor should you.
Neither should you be comparing your work in progress with other writers, for example, in a workshop group. You're not writing their book, you're writing your book. Their book reflects them, your book should reflect you.
And the other thing it reflects is the amount of writing in-put the book has had, in terms of experience, inspiration and time. They might have been able to spend more time on that piece of work, or been in the process of writing it for longer. Perhaps they might be near completion of the story and are utterly certain about what's happening and where it's going, while you're still floundering.
So don't be demoralised by reading other people's writing if you think it's miles better compared to your WIP. Yours can get there too - if that's where you want to go. It might not be what you're doing, or what you want to do. I can admire and enjoy someone's magical realism writing, but it's not what I want to write.
It's a bad idea to compare your writing to others' work, or even your own previous work. I'll write about comparing yourself to people who get published even though you think you're a much better writer next time.
But for now, all comparisons are odious and may de-rail you. Just keep on going and write the best you can write right now.