Have you ever said to yourself, "I'm too close to this material to be able to rewrite it?"
That's exactly the problem, so the answer must be...distance!
There are (at least) three ways to get distance from the material in order to be more objective about it.
1) The distance of time. If possible, put it aside and start work on something else. Give it at least a week or two, ideally a month. Working on something else in the meantime is crucial, because that new project will help you cleanse your mind so that when you return to the first one you'll see it much more clearly.
2) The distance of space. When you critique your own material, don't do it in the same space where you composed it. In that space, you get into a creative trance. Now you want to snap out of that and be able to be analytical. Print out your project and take it at least into the next room or, ideally, a totally different location like a coffee shop.
3) The distance of a different sensory system. When you write, you hear it in your head. Now read it out loud and hear it that way. This is essential for material that is intended to be performed, like plays and films, but it's also helpful for novels and short stories. You may even want to get someone else to read it. If you find that too embarrassing in your presence, have them record it and listen to the recording in private.
You can experiment with all these and find out which one is most effective for you.
(there are lots more tips on writing and rewriting in my book, "Your Writing Coach," available from Amazon and other online and offline retailers. More information at www.YourWritingCoach.com)