I couldn't read this without hearing the Genesis song "Dreaming While You Sleep" in my head. The song's narrator hit and killed a young lady during a heavy rain, and was wracked by guilt much as you were:

All my life, you lie silently there,

All my life, in a world so unfair,

All my life, and only I know why,

And it will live inside of me,

I will never be free all my life,

Trapped in her memory all my life,

Till the day that you open your eyes,

Please, oh oh oh, open your eyes...

Maybe this helps, that other people have captured the kind of feelings you're feeling. You're not alone.