I am

[TheWall0912]

the unborn witness of birth and death.
 I really like this opening

The surface of my breath is separated into boxes: souls growing, slow,
And in my shadow-stuffed hollows, I swallow my souls alone.

 I'm not sure I fully understand what you're aiming for here

Inside, they know,
I am the one who loves them so. Decent ending for the point you're trying to get across. The narrator is God? Or some godlike figure?

I like the idea you have here, but I too think it needs some editing. I think the poem would be more effective if S3 also began with some form of "I am." But I also tend to be too repetitive in my poetry... so that's just my opinion. Looking forward to another draft!