Contrary to urban legend, editors are not actually out to corrupt your artistic vision. They’re out to make your book better.
Everyone needs an editor. And when I say “everyone,” I mean everyone. Big authors, small authors, and everyone in between. No one gets a book published without getting it edited, unless they’re self-publishing (and even then, it’s wise to use freelance editors). The reason for this is, you can’t edit your own book. Even professional editors can’t edit the stuff they themselves write. You already know all the background information, the twists and turns, the motivations. You’re simply to close to it.
A writing group can help you with the craft of writing, but an editor has a different focus. Unless you’re in a very specialized writing group, your talented group-members probably don’t know your audience like an editor does. Hell, you may not even be sure who your audience is. And that’s perfectly fine. That’s your editors job.
Your editor will know the conventions of your genre, and how to break them, and how they’ve been broken before. Your editor will know who you’re competing against, and how you’re different. Your editor will now who wants your book, and how to get it to them. They’ll know how to help you through the hard edits, the rough reviews, and the sleepless nights. Your editor is out there to make your book the best that it possibly can be.