Our eyes and minds play tricks on us when we read our own writing. We “see” what we think we wrote, what we meant to write . . . not always what’s actually on the page or the screen.
To catch errors such as missing words, try reading your work aloud. This forces you to notice each word. It’s also a great way to check sentence structure and to assess the rhythm and flow of your writing.
Also, have other people read your writing. Another pair of eyes is invaluable. I’ve been editing other people’s writing since 1986, but I hired an editor to review new material for my Fitzgerald book. She didn’t find many errors . . . but she found a few things I would have caught in anyone else’s writing but missed in my own.