I only read your post today. I agree with everyone else it was just an accident, don't blame yourself (even though if it happened to me I would be just as devastated as you).

On a busy road like that it is very dangerous to stop, I'm sure that if the circumstantes would have been different you would have done so. Not only, but you did everything you could: you alerted the police and were willing to leave your details. You have done so much more than a lot of people would have done.

Please take care of yourself, you will get over it.