Perfectionism vs. Product in Writing (or, Just Turn the Damn Thing In!)

Do you struggle with trying to make your writing perfect? Gosh, I do, and it is the bane of my existence.

It's what makes me a good writer and what makes me a terrible writer.

The older I get, the more I realize that what makes you good also makes you crazy and bad. The same exact thing! WHY IS THAT???

If only it were easy to know when your own writing is good. If only we didn't get numb to our own work, that after so many passes through, that we lose all judgment, good and bad.

How do you combat this problem? Please tell me!

Throughout the marathon of writing a book, mental endurance takes on many forms.

--You have to sustain the story. It has to be a story, with plot points, twists, turns, and classic dramatic moments.

--The characters have to act like real people, in all their challenges, quirks, relationships, etc., and they have to act like the same people--your heroine can't be calm and laid back in chapter one then Type A everywhere else. They must grow and fight against change and ultimately take the leap of faith that transforms them into their best selves.

--You have to sustain your reader's interest.

--And you have to sustain the quality of writing.

The point is, it takes many interwoven threads to make a book, and as the author, you not only have to keep track of each one, you have to perfect each one so they all blend together seamlessly.
A Herculean task. A true mental marathon.

It would be so simple if it weren't for one small problem: the fact that our own brains trick us.

The big problem for me is that my brain is fickle. One day, I can read a paragraph and think, did I write this? It's actually good. But just wait a week, make another pass. Then I spot all kinds of really awful things--stilted dialogue, really sucky metaphors, flat ending hooks that I once thought were clever--and am inclined to say, this writing really sucks.

The result is a viscious cycle of not having a clue if anything's any good or not.

Yes, I have people critique me. I have people read my stuff. But sooner or later, you have to stop reworking a story and say I"M DONE. 

But how do you know when that is? How do you know when to just say--that's it. Nada mas. When do you stop fiddling with the dialogue to make it more natural, when do you quite searching for just that certain perfect metaphor/word/sentence/ending line that makes the scene perfect?

When do you ever let it go?

Can anyone relate to this, or am I way off the bell curve in the realm of people who enjoy tormenting themselves to death?!