I just saw this thread, and was hoping Missy was going to be ok! I am very sorry she had a bad night and in so much pain

Poor little Missy. your vet will take good care of her, and make certain she will not be hurting .
This is not your fault though! My Suzi was just like that as a puppy always trying to jump out of my arms outdoors! It is very heartbreaking when they get hurt , however they are wiggly and tiny, very fast and impulsive. PLease do not blame yourself.
I hope for the best for Missy, and the vet won't have to do anything evasive. my thoughts and prayers are with you and Missy .