Am unable to find any such thing by a famous poet in the way the you are describing. There are these:
"[The Godfather] got much better reviews than I expected. I wished like hell I'd written it better. I like the book. It has energy and I lucked out by creating a central character that was popularly accepted as genuinely mythic. But I wrote below my gifts in that book."
~ Mario Puzo, "The Making of The Godfather," The Godfather Papers and Other Confessions 
"I think I could have written it better, assuming, of course, that I had the leisure for
the study he has put in.”
~ Mahatma Gandhi (referring to Karl Marx’s “Capital”)
"Three years ago in Paris I got out of a "metro" train at La Concorde, and saw suddenly a beautiful face, and then another and another, and then a beautiful child's face, and then another beautiful woman, and I tried all that day to find words for what this had meant to me, and I could not find any words that seemed to me worthy, or as lovely as that sudden emotion."
~ Ezra PoundThe apparition of these faces in the crowd;
Petals on a wet, black bough.
Started as a 30 line poem, then edited to a 15 line poem, and then finally down to 2.http://www.helium.com/items/1332415-in- ... ezra-pound