I know this is a common saying, but I'm just now experiencing this for the first time; in all the many hours of self-editing. They say to edit yourself as though you were editing the work of someone, anyone, else.
You don't want to know what the words are supposed to say, or the impact they are supposed to have.
I'm working through another draft of something I began writing more than twenty years ago. And I'm like: That's not the right word. And that's not where you use a semicolon. And WTF's with all these commas?
I'd be more than a little embarrassed, if I weren't having so much fun fixing things up. Self-editing isn't easy, but it should be satisfying.