During NaNoWriMo, and any other time you're writing, the biggest danger is trying to be perfect. Perfect is an enemy of finishing things. If you keep going back and fixing every tiny error or imagined error then you will never finish anything.
It's so easy to get caught in the 'everything I write is crap' loop that you can forget to move forever towards THE END. Reaching the finish line is the goal here. You can't edit a blank page.
This is why I like NaNoWriMo so much. It's a great way to force yourself to focus on reaching the end and not focus so much on making something perfect. When you're dealing with 50,000 or 100,000 words you've got to finish telling the story before you start worrying about if the commas are right or if if maybe you should change the opening scene.
Be like a shark and keep moving through all the worry and panic. Focus on nothing but geting words on the page and finishing everything you've got rolling. Reaching the end is what matters, then you fix it once you've finished everything.
So keep writing and running for the finish line.