Oh, Nicole. I'm soooo sorry to hear this. My heart breaks for you. My brother and his family lost their Australian Shepherd x Great Dane puppy the same way, only it was a plastic grocery bag. They were devastated, too, of course. If nothing else, we have all learned something from the tragedies that have been shared here.
I know it's easier said than done, but please don't be too hard on yourself. I send you hugs and comforting thoughts, and I hope the support of your friends here is of some help.