Wise words from Terry Pratchett! It's easy to beat yourself up when you're writing a first draft, but I'm a big advocate of "pen to paper." Just get your thoughts out!
When I first started writing, I would rewrite myself over and over before moving on to the next passage. Now, I'm working hard to keep it moving! If I need to research something, I jot a note or make a highlight. So, if you ever run across any of my drafts, you will see crazy scribbles like "insert cool name for candy that changes color when you eat it" or "research body language of an angry giant."
When the words flow, sometimes I even surprise myself! Happy writing!