The strong anaphora of the first five lines is cut suddenly by a deviation from that form, as well as a deviation from the action being taken: the strong iterations of what you won't do is undermined by the single admission of what you will do. The image of a razor brings a bite of physical pain along with the emotional pain implied throughout, but is not a good solution, as the poem suggests: it is only for one night. Any more than that would be detrimental to the healing process--prolonging the pain does not make it go away.