Resist the urge to stop and go back. Finish what you start, then re-assess.
I agree with a lot of the stuff in this thread, but this last point is super important! And it's one I've learned the hard way!
I recently finished a first draft of my first novel, and the only reason I did was because I didn't keep going back and fiddling with stuff like I did on every project before that (all of which died on the vine, semi-complete and over-polished). I only ever looked back as far as the previous day's writing. And honestly, keeping as close as I could to a daily word count quota kept my head in the story enough to remember enough from the parts I hadn't read in a while to grow interesting things from seeds I'd planted (both on purpose and by accident) in the earlier chunks. I'll stipulate that I'm a pantser by nature, who only needs a bucket of ideas about character, setting, and events to get me started.
But now I've started a first draft of another novel, and am feeling those initial "in over my head" gasps that come with starting a novel. Yesterday I was nearly done with what I think will be an important introductory scene, and I was concerned about where I'd go from there. The idea came to stop once I'd finished the scene, and reread and tweak things "until I'd learned enough about the character to make an informed decision." And thankfully my subconscious was like, "What, are you nuts?! Have I taught you NOTHING?! Full steam ahead!"
That said, I will be adding bits to my character docs based on what I've learned about the characters and their predicament, but those will be understood as
notes, not taken as
canon, since one of the most wonderful things about a first draft (or "zero draft") is that there
is no canon.
It's like starting out SCUBA diving. Those initial gasps of "I can't breathe down here" are important to acknowledge. But you have to keep calm and keep writing. There could be a great story down there!