Here is a test to determine if you need an editor:
Cna yuo raed tihs? Olny 55 plepoe out of 100 can. fi yuo cna raed tihs, yuo hvae a sgtrane mnid too.
i cdnuolt blveiee taht I cluod aulaclty uesdnatnrd waht I was rdanieg. The phaonmneal pweor of the hmuan mnid, aoccdrnig to a rscheearch at Cmabrigde Uinervtisy, it dseno’t mtaetr in waht oerdr the ltteres in a wrod are, the olny iproamtnt tihng is taht the frsit and lsat ltteer be in the rghit pclae. The rset can be a taotl mses and you can sitll raed it whotuit a pboerlm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe. Azanmig huh? yaeh and I awlyas tghuhot slpeling was ipmorantt!
Did you pass? Congratulations! While this version claims that the ability to read this is a slightly below-average skill (fifty-five out of a hundred means that most people can do it), other variations declare only geniuses can read it. So again, congratulations, but unfortunately, it also means that you might need an editor. It is astounding how easily everyone--including "geniuses" like you and me who can read the message above--can adapt and overlook simple mistakes. This problem only grows when applied your own writing. You know what you mean to say and your brain adapts to see it. Suddenly, there are no spelling mistakes, dangling modifiers, or gaps in logic. Suddenly, your Civil War character is dying in 1683 instead of 1863. Suddenly, the difference between a restricted and unrestricted clause disappears. When this happens, your writing suffers and so do your readers.
Don't be embarrassed; this happens to everyone who attempts to write. Once, I sent off a piece to be published, but it came back rejected. The rejection offered no feedback on tone or pacing or argument, but, instead, had one long comment from a reviewer pointing out I routinely replaced "from" with "form" and how that mistake made the piece a chore to read. You are already ahead of me. At that point, I had not even asked if I needed an editor.
Take a tip from the pros: hire an editor. Professional writers know that an editor is the difference between good intentions and excellent execution, or, put another way, rejection and publication.