"
But you're the one who shot me
And goddamn it, I can barely
This part means, after he decided that she was his everything, and that no one could take her away from him. She called it off, and left him. He was crushed and felt like his life was completely over, and that there was no point in living. So she is both the gun and bullet. The gun because she doesn't want to shoot him. But does anyway. And the bullet for living him damaged.