So how do you manage rewriting if you don't want to quit? Here are some ideas...
1. Get some outside input. Worked for Tolkien! Use a workshop, post online, or, if you intend to publish, start looking for an agent. If you get one, let the agent make recommendations, and an editor thereafter. Remember Heinlein's Law: "Only rewrite to editorial order."
2. Set some structure around yourself; once you've reached a point where you think it is good enough, start sending it out or posting it or whatever, and start something else while you wait for a response. Force yourself to move on!
This is difficult, because it is linked to your motivation. It's one reason why I love this quote from W. Arthur Rydee:
"A masterpiece requires two people--one to create it, and one to stand behind him with a club to hit him when it is finished."
Words to live by!