The same thing happened to me some years ago. The driver was a 16 year old who had just gotten his license a week earlier. The dog, a pretty German shepherd was not maimed like your dog, but died slowly. It took him about 20 minutes. The family that owned him, a mother with 3 or 4 young children, witnessed it. It was all very sad. It took me a while to feel right riding again.
I"m sorry that this happened to you. Remember that it was not your fault.
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Religion is a good thing for good people and a bad thing for bad people. --H. Richard Niebuhr