That’s the secret to first draft writing. Just keep plodding forward, let the momentum build. Put the screen down on the computer if you have to. Just keep writing and writing until you forget it’s you at the keyboard. Finish for the day then start where you left off in the morning. But whatever you do – Don’t look back!
No till the end anyway, when you can scream, but at least you’ll have a whole first draft to fiddle with, no matter how ugly.