Sad to want to tell you I know but, I have been out with a client this afternoon with a very "challenging" rescue dog. Initially you could not touch her, or look at her, she was aggressive towards people, dogs, leaves(!), cars anything.
Today after a lot of build up sessions she was running free in the paddock with all of my dogs, wagging her tail, chasing, playing and just being a dog. I love my job!!!
Client was in tears - of course I was professional and sobbing quietly 
Never give up on dogs - despite whatever has been dealt to them with positive training they can do anything