7. You’re a lousy judge of your own writing. (Maybe what you wrote is just fine.)
No matter how shitty you think a first draft is, you can fix it. Think nothing can fix your third draft? Get someone you trust to read it.
The big benefit (maybe the only benefit) of working with a professional editor at a major publishing house is that your editor can tell you what’s wrong with a piece. You may complete a book, and think: “there’s something wrong…”. You sense that you’re not hitting the mark, but have no idea how to fix it. Your editor will tell you.