Here's a truth that freed me: your first draft doesn't have to be good. In fact, it probably won't be. That's okay. The real writing happens in editing.
When you finish your first draft, set it aside for a while. A day, a week, however long you can manage. Distance helps you see your work more objectively.
Multiple Rounds of Editing
Good editing happens in passes. First, focus on structure. Does everything flow logically? Then look at paragraphs and sentences. Finally, check for typos and grammar.
It's hard to do all these at once. That's why separate passes help. Each round has a different focus.