What to do to ease this guilt of my kitten dieing?

I have a cat thats almost a year old. Love him like crazy.. My husband found a dead momma cat and three kittens. They wer bout two or three weeks old.. My husband brought one home for me to bottle feed and to take care of it. I did that for bout 2 weeks. Got attached to him. I was feeding him then i sat him on his blankie on the floor. I went to the bathroom. My 3 year old son accidently stepped on him. He looked ok. I called vet they were closed. He cried but nothn came out he didnt meow. 15 minutes later. With me holdin him he died i tried mouth to mouth but nothing.. Im so heartbroken. I cant believe i let this happen.