Most of us edit a few pages then stop to admire our work. After that we resume editing -- but we return to the beginning... again and again and again.
Result? You're editing the opening over and over while the rest gets short shrift.
Do this instead: print it out. Go through the whole thing with a pen and mark what needs attention, but don't make the changes. Just circle 'em.
When you reach the end, *then* go back and make the changes. Do it only after you've circle-marked the whole thing, not until.
Now your entire document gets serious attention, not just the top.
(Sleeping puppy -- name, Mark Watney -- pictured below in shameless attempt to draw your attention.)