The subtitle, "after the waterboarding," tells us where the speaker is speaking from: someone who has been tortured and has already given in. The first three stanzas repeat "I was ready," and each time the speaker would accept something awful, whether confessing dark thoughts, being blown up, or hanging, just to escape the sensation of drowning. What they crave is simple: air, steadiness, the pull of the ground.
The turn comes when the speaker admits that the "promise" that broke them was only that the "atrocious weather would change." That plain, almost mundane phrase makes the cruelty feel bigger, since the relief offered was so small. Then comes the quiet, devastating ending. The weather did change, and so did the speaker, into "something I do not know." The mood is bleak and stunned rather than loud. Surviving hasn't brought relief, only a loss of self.