The poem
IN a drear-nighted December, Too happy, happy tree, Thy branches ne'er remember Their green felicity: The north cannot undo them, With a sleety whistle through them; Nor frozen thawings glue them From budding at the prime.
In a drear-nighted December, Too happy, happy brook, Thy bubblings ne'er remember Apollo's summer look; But with a sweet forgetting, They stay their crystal fretting, Never, never petting About the frozen time.
Ah! would 'twere so with many A gentle girl and boy! But were there ever any Writhed not at passed joy? To know the change and feel it, When there is none to heal it, Nor numbed sense to steal it, Was never said in rhyme.
This version is the 19th-century one, probably an editor's invention (though it could be a variant tried by Keats). See "In drear-nighted December" on this site for a version now more generally accepted as Keats's own. The important change is in line 21: "The feel of not to feel it" rather than "To know the change and feel it".