"Your goal is to make the book as good a book as you can possibly write at this stage in your life. And you don't stop until you have done that, and when you have done that, it's a mistake to play around with it because then you'll just succeed in ruining it.
"You begin to sense the point at which you have done as much revising as you can do. It's not exactly right, you haven't served it as well as it should be served, but that's as far as you can go."
It's hard to let go, to turn your back on something--a poem, a painting, a performance, a position, a relationship--when you know you've not served it as well as it should have been served, when you know that you could (an abstract and non-specific could) have done better. Unfortunately we operate in a very concrete and specific world in which our efforts are wildly variant in their effectiveness. Time of day, the effects of our social contacts, our physical health, broken shoestrings, lost keys and an endless plethora of other physical and emotional factors all put our coulds in a state of wild flux. Our best today was not our best yesterday and will likely not be our best tomorrow. Accepting that is hard. It requires admitting one is human and fallable, and for some of us this is much harder an admission to make to ourselves than to anyone else. Vulnerability usually is.