Do you need an editor? Yes.
You may not think you need an editor, but just know this: you do.
Even if you’re a “good” writer, no, a great writer, you still need an editor. In fact, dare I state this:
The greater the writer needs the greater an editor.
I’m sure that will go down in the quote books … but what comes first? Great writing or great editing? Or does great editing make good writing great?
It’s also just another pair of eyes on the work. Ideally, those eyes have other strengths: grammar, flow, story, etc. It could even be, gulp, fun. My mother (master of language, grammar, and story wrapped together in one mensch) just helped me edit The Secret of Kite Hill. There were grammar mistakes, lots of places that needed commas (I find commas challenging … ) and some general storyline work.
It’s great practice to get rolling with an editor on an early book to get the process down better, too. Don’t wait for your masterpiece to start working with an editor. Have an editor work on your basic, run of the mill work, to see how you work together and how it might work later on. Maybe it’s a bust. Maybe you’ve found your writing soulmate and you’re set for life. Maybe they just add a few commas. Whatever happens, you need an editor. No, really.