When it comes to writing a first draft, the key is to avoid obsessing over each and every line.
If you feel your perfectionist tendencies kicking in, you need to be on high alert. The urge to rewrite every sentence you put down is a normal once; nevertheless, it should be closely monitored. If you focus on making your first draft sound perfect, you're going to get discouraged and might feel like giving up.
Here's the best advice that I can give you for finishing first drafts: Don't get it right; get it written.
What am I talking about?
When it come to writing a first draft, avoid editing as you write. The focus shouldn't be on making everything sound perfect--not yet, anyway--but on getting it all down so that you can fix it later, once you've moved on to the second draft.
You'll never finish what you start if you obsess over every little detail that you're putting down on paper. You want to finish that first draft? You need to get it down as soon as possible. Don't worry too much about how it sounds--you can always fix that later.
That's what second drafts are for.
Dump everything from your brain onto the page without hesitation. I promise you'll be much happier when you're not self-editing every step of the way.
Don't get it right; get it written.
What do you think about editing while you write? What do your first drafts typically look like?