A first draft is a piece of writing that sucks.
This is the most liberating insight of writing. When you find yourself getting hung up by your internal editor while you write, or when you feel discouraged while revising your first-draft prose, consider:
1) How delusional is it to expect flawless writing to burst forth from your brain on your first try?
2) Wouldn't it be unnatural if your work didn't need several editing passes before being ready for public viewing?
3) Why would you expect your first draft to be perfect, or even remotely good?
All first drafts suck. Get over it. And the next time you sit down to write, recognize the truth of what you're about to do: you are going to write a first draft that sucks.
Then shut up and write.