It's not your fault she got run over.
It is very hard when you leave a healthy animal and come back to find them gone - it seems easier if they are sick or old first.
You did a wonderful thing by taking her in and loving her. Friends who spend time with us fleetingly are usually sent to teach us something. At some point you will learn what that lesson is. In the meantime, don't blame yourself, it was her time to go to the rainbow bridge and nothing you could have done would change that.
Only As Old As You Feel - Sometimes I Feel VERY Old