[Colwell is examining the various drafts of "One Art," and focuses here on Bishop’s struggle to write the last stanza.]
… [B]y embodying uncontrollable emotion in a form meant to control it, and in an utterance meant to deny it, Bishop can create tension, ambivalence, and a poignant recognition of the pathos of human attempts to control the uncontrollable.
In the earlier drafts of this stanza, Bishop struggled with the desire to say and unsay, to say two things at once, both admitting to the truth of the argument that the villanelle has established and admitting to the evasion of the truth that the tone has insisted on. To accomplish this she tried lines such as "of course, I’m lying" and "it’s evident I’m telling the truth"; one draft of one verse completely explodes the villanelle form:
All that I write is false, it’s evident The art of losing isn’t hard to master oh no anything at all anything but one’s love. (Say it: disaster).
This duality that Bishop works so hard to achieve in draft after draft (there are seventeen drafts of "One Art" in vassar’s manuscript collection) she finally finds in one word, "shan’t." This word, with its overformal stiffness, its anachronistic sound, its school-marmish precision, says both "I’m lying" and "I’m not lying." Using the future perfect tense allows an ambiguity that no other grammatical structure can provide. Bishop accomplished the feat of expressing her ambivalence about her own endeavor; in the word "shan’t" she combined opposite meanings in one utterance.
From Anne Colwell, "Geography III: The Art of Losing," Chapter 4 in Inscrutable Houses: Metaphors of the Body in the Poems of Elizabeth Bishop (Tuscaloosa: University of Alabama Press, 1997), 178