Someone--not a writer, interestingly enough--once told me she thought writing must be like having and raising a child.
You give birth to this beautiful creation. It's a painful and wonderful experience. As it grows, you discover that yes, it has its flaws--but who doesn't? You try to make it the best possible thing you can make it, shaping it and molding it as time goes by. You always love it, no matter how much it sometimes frustrates you.
Then someday, you have to just...let it go. You have no choice. You've done all you can do, and--no matter what the flaws--you take pride in your creation.
When she told me that, I thought about the summer before I left for college. It seemed like my mom suddenly had all these things to tell me that were VERY. IMPORTANT. I realized, eventually, that she was getting ready to let me go...and just wanted to get those last bits of parenting in before she did.
Just like, before you send that MS off to an agent, you think, "Wait! Maybe I should run through it...one more time...."
P.S. As much as I think the metaphor fits, I do not advise spending eighteen years editing the same work.
