I once read--I no longer recall where, or in what context--that there is no way around grief; there is only through it, and out the other side. The trope of breath and breathing is perfect, because grief is, in its essence, so much *like* breath. It can be ignored--held, suspended, denied--for a time, but inevitably it forces it way back and must be dealt with.
Your story is a painful, beautifully nuanced illustration of that truth. I don't often read character-death stories, particularly those clearly labeled 'angst', precisely because they are painful, and there's enough pain in life that I don't often choose to seek it out in my leisure time.
And yet.
I cried through most of this story, and I expect it will be one of those I think about off and on for a long time, and that will make me cry again. It is also quite, quite beautiful, and compelling, and true. I can believe this version of Harry, struggling with his loss, which he believes he cannot share, trying to pretend Snape meant nothing to him. I can believe this version of Snape--and my heart breaks for him, even more than for Harry, because I *do* think he loved Harry, to the best of his broken ability. Enough, certainly, to walk away, taking only a memory "he will only access after too much Firewhisky." Harry has Ginny, and his children, and his friends to sustain him. Snape had Harry, and he let him go, and well, there, now I'm crying again.
Ginny is *amazing* in this story. She is so often ignored--or, much worse, pettily denigrated--in fanfic, that it is remarkable to see her as a fully-realized, even admirable character. In this story I can see why Harry would love her, and can believe in them being happy together, really a remarkable accomplishment, especially in a Snarry story where I can *also* believe that Harry loves Severus, however much he tries to deny this as a way of protecting himself from grief.
The ending is perfect, trying back to the breathing image, and showing very clearly the way time, events, a life, can be defined by grief, the way it is a break as complete and disorienting as a the snap of a cable, the way that cable, that life, can be knitted together again, and be strong, but not the same as it was before.