Oh wow, that is something I can imagine will never leave your brain, that is as horrific as it gets when it involves a kid, I’m so sorry, I can’t even imagine it. My cousin was a cop in New York city and actually a good friend of mine was for 2 years who went on to join the secret service. He was a combat vet for operation Desert storm and all it took was for him to witness one subway accident victim and he resigned and went to the secret service.
I did see a lot of really nasty and heart breaking stuff in 25 years. There is a cumulative stress that begins to wear on you after a while. You finally realize that a job that you once loved is not something you enjoy any more.
I always thought on days when the real action took place that it was a little like watching a movie only I was really there. I am not sure that it is really that much worse for you than someone who watches a whole lot of movies, documentaries and TV shows with awful scenes in them. Maybe the difference is that in shows they usually write in some type of ending that wraps thing up for people...
I usually do not think much about the very sad things that I saw.