What are the differences between using an editor and self-publishing for a first book?

When you self-publish, unless you hire a freelance editor, the responsibility for editing your work falls on you.

Beginning writers almost always think they write better than they do. It’s not surprising because, as with most skills, we improve with practice. Getting the benefit of professional advice also helps. That’s what coaches are for in sports.

I’m not self-published. After eleven novels (the last is in the final stage of editing), my editor still finds things that need to be changed. I’ve come to realize that by the time I submit it, my story is so familiar to me that my brain doesn’t always “see” the places where I’ve messed up the timeline or put the same paragraph in two different places or contradicted something I said earlier. I’ve learned to let the ms. sit for three weeks or a month before going over the final draft before sending it in. That helps, but she still finds problems. I’ve also learned a lot from her.

Even people who can write a really good story will not necessarily do a good job of editing or proofreading it.