Stop looking back. DO NOT EDIT UNTIL YOU ARE FINISHED.
I don't care what you're writing! A novel, a play, a short story, a poem, do not look back until you reach the finish line! There will be loads of time for you to tweak every detail later, but you have to train yourself to just push forward until then. This is something I have an issue with, and nearly lost a very big project because of it. I managed to quit looking back at it, though, and I'm almost done now. But trust me, when I finish, I am going to go back and rip it to shreds. Then I'll neatly reassemble it into this thing we call a novel. Until you are done, though, don't let yourself get caught up in what you've already written. Just stay focused on what you're writing.
You end up with a file, at least. While my inner editor interferes already while I am still in the planning phase. Not realistic enough, illogical, Gary Stu, RP-writing, wrong details, stereotypes, clichés, plagiarism. I see it everywhere. When there is nothing to worry about. For some reason it is easier for me to write something when I don't really care and just mess around for fun.
What helped me about a year ago was taking notes by hand. When I write on paper, it somehow is easier. My notes look very messy, but it still is some psychological trick, because I am able to convince myself that I can fill in stuff and fix things while typing.
Right now I work hard to see the positive. Unintended plagiarism (similar character traits or scenes) always happens, it also means that something actually does make sense and will be interesting for some readers. I am also telling myself that this story project is what I would want to read, that my protagonist is exactly the person I want to meet in a book. Or movie. Or what ever. I am telling myself that tropes are not clichés, archetypes are not stereotypes. That not every tiny little detail has to be correct from the beginning. That even language inconsistencies and character voices can be fixed later.
Time to kill our evil b****es! Let's get the drafts down! Now!
I take extensive notes by hand, but that's all they are: notes. For some reason, I can't sit down and write a story by hand. I have to sit in front of a computer and then the story comes out. But in notes, I can write down what I know is happening.
I'm suggesting this mostly to play devil's advocate, but Embrace your inner editor. Come to the Dark Side! Let the compulsion to edit flow through you!
Being serious, though, I find myself doing the same thing. There will be days where I barely write anything, and instead just pick over what I did the day before, or the day before that. The thing is, I feel like this helps the writing overall, and pretty soon I'll either decide that what I've got is okay or I'll get tired of editing. Whatever happens, I'll then go back and continue the story. I think this sometimes even helps me include new ideas or keep characters consistent. For the record, though, I'm also doing NaNo this year and I'd definitely be having an easier time if I wasn't still thinking about quality.
Lots of people recommend not worrying about how good the writing is and just getting it done, and I think this is fair advice. The problem is, it's become so universally accepted that it's liable to turn into a problem in itself. "If your writing isn't perfect to begin with, you're not a good writer" may just turn into "If that first draft doesn't come together right away, you're not a good writer." Neither is particularly encouraging and neither will help you get the thing written.
All in all, it seems possible that you might get further not by doing anything differently, but by looking at it differently. If you saw all the editing as effort put towards the finished story, might you not have more motivation to carry on working? It's not a guaranteed solution by any means, but I think it's worth considering before starting any struggle to "turn off" your inner editor.
On a totally different note, #WritersInk runs "Write Offs:" chat-based challenges where writers come up with a story/poem in just twenty minutes. Since there's no way you can possibly do any serious editing in that time, it might help you learn to just get words on the page.
Admittedly, that is how I naturally approach writing, but I don't like to recommend it just based on that. If I wanted to churn stuff out like Stephen King I expect I'd have to learn to draft faster and edit better. And I can't be sure that it wouldn't produce better results, too. One of the problems with editing as you go is that you sink quite a lot of time into fairly small bits of writing, so there's a real incentive not to scrap them (ever) even if they're just not working.
Yeah, it seems for NaNo that the only way to reach the goal on time (and at the everyday markers) is only possible if you don't focus solely on quality. /like 15k behind
I think another problem for writers, or just anyone who has to write anything in general, is that schools at the very least (or maybe it's just my area) are stressing the need for success and achievement. I know my English classes stopped requiring drafts years ago, so we'd all turn in papers and more often than not get marked down for simple mechanical errors and disorganization or something. Assuming writers attended schools in this kind of environment, I could see how that mentality of having everything perfect the first time around could stick onto other aspects of the person's lives. Like, maybe artists have the problem, too, where they want that first sketch too be a masterpiece. I dunno, just a thought..
I saw it a while ago and I've been waiting for an opportunity to use it.
I've mostly been getting through NaNo so far by sinking an awful lot of time into it. Personally, I think that as long as you're not using the "cheap tricks" they recommend (chapter summaries included in word count, pointless extended flashbacks at every opportunity) then you've still managed some kind of quality. Getting to 50k is good, but I think the achievement would be marred by the knowledge that your first edit would involve cutting out all the stuff that wasn't actually part of the story.
School really might have something to do with it. At the very least, you're expected to have homework and such polished and ready for reasonably short deadlines, and you save a lot of time by making your first draft more or less your only draft. At my university they did recommend just "getting it done" quickly and then editing later, but even so I don't think it would have been practical to approach it like NaNoWriMo, where you don't need to think about quality at all the first time around.