something can be marvelous and still need to be stopped

I've never had much problem having a sense of where a movement ended and where I wanted to break a stanza. Though what happens is, you learn to write a poem that breaks stanzas in a certain way, that takes certain kinds of linguistic, syntactic turns to stand for closure. You recognize that turn as closure, and as soon as that moment of recognition happens, then you've got to stop doing it. Because then what you're doing is simply making of everything the same poem. You look at a tree, and you turn that into a tree poem, and you look at a rock, and you turn that into a rock poem. They all have the same arc. As soon as you can recognize a consistent shaping principle, recognize that a certain kind of sentence is always a cue to you for an end, then you've got to resist the cue.

...

Something can be marvelous and still need to be stopped. Otherwise you don't change. It's as simple as that. And if you don't change, then you stop writing good poems. Really. I believe that. So when you can identify something as a maneuver, however successful, even if you never do it badly, you should stop doing it. 

...

I found writing [my new poems] that I had much greater difficulty knowing when something was good and something wasn't good. This is always true when style changes. Because your editorial judgment has been honed to a particular method. The tools you have for recognizing errors and intentions in that method are no longer of any use. So you have to develop a whole other critical sense. And the transition is difficult, because you usually have very little sense of whether or not you've written anything - I mean, you keep thinking, This is absolute trash; this is trash. But you've got to write it.

...

If I have any message to any of you who write, it's that you cannot sit calmly repeating yourself. And the hard thing about that is, oftentimes when you change, the new poems, the adventurous poems, may be less successful than the evolved poems of another mode. So you have to be willing to set aside that degree of finish, that degree of polish, for something that might seem primitive. 

Now, not all such transitions work that way. Occasionally you will discover a new kind of language that will seem to you an advance. Or will make the other poems look thin. Which is I guess how you experience advance - the rug pulled out from underneath you; it's never completely positive; if you like what you're doing, then it makes everything else that you did look crummy.

But oftentimes it won't happen that way. Oftentimes what will happen is that you will find yourself writing less accomplished poems, and you have to be willing. Because there's no other choice. You can't go on writing poems in those rehearsed ways... As soon as expectation begins to form around your work, either on your part or on the part of readers, you must do your best not to gratify it.

- Louise Glück, in conversation with Pearl London in 1978, from Poetry in Person: Twenty-five Years of Conversations with America's Poets (ed. Alexander Neubauer, Knopf, 2011).