Castiel looked up at the sky, blinking back tears that were threatening to stream down his face, trying to will the stars back into focus. The words to the song Crowley had sent him as a vile joke swam around his head, flinching when he realised how true they really were. He really wished Sam and Dean were there with him to comfort him, but a hunt had called them away, leaving him to take care of the bunker. As the stars came back into focus, he thought about his brothers and sisters falling and one line of the song shouted out to him, almost like the brightest star he could see, the only thing he was really sure of.
...If heaven's grief brings hell's rain...
...Then I'd trade all my tomorrows for just one yesterday...
This Castiel was sure of. He blinked again, wiping away errant tears, and sending a silent prayer to his siblings even though he doubted they could hear him. He was sure that he would give up any tomorrows he had to fix what he had started, to get the angels back to heaven.
And as the first star fell, the words rang true in his head again, causing him to hum the tune to himself as a silent promise to fix what he had done.