This is the whole paragraph I’m having trouble with:
His uncle backhanded him so hard across his left cheek that Isaac’s sense of self temporarily broke upon the ground on which his body fell. He lost any feeling of personal cohesion. And as the parts of his psyche put themselves back together, some secret piece of himself remained in the dirt, left behind in a place from which it would never be recovered.
I don't understand the meaning of "self temporarily" in the first sentence. Isaac's uncle backhanded him, but what happened next? And what is the meaning of "broke upon the ground"? What is that verb’s subject and what form is it in?