Yes, you need an editor!
Because no matter how good your writing is, you're too close to your work.
Because no matter how good your grammar is, no one knows every rule, not even professional editors. But they do read day-in and day-out, and they do know what typical errors to look for. And again, you know what your sentence is supposed to say, and that's what you will see.
Because no matter how much research you do, you may have an erroneous fact. Professional editors help with more than just grammar issues. They will check facts, look for common--and uncommon--mistakes, misspelled character names (yes, this happens,) changed characteristics like eye or hair color (yes, this happens,) or what time it is in Beijing or Afghanistan when it's 2:00 in New York. (Yup. That happens, too.)
We offer manuscript editing and proofreading for self-published authors, or anyone who wants a final read before submitting to an agent or publisher.