I think one the problems we have here is definitions, what are we dealing with - shellshock/battle fatigue/post trauma stress/stress/tiredness. Others in this thread have better expressed this point than I have. I am no expert but I would be surprised to find that people in the 19th century did not suffer mental trauma after a traumatic event just like people in the 20th/21st centuries. What does seem to be different is how these things are reported & written about. Maybe it was considered more of a weakness than now, best not talked about or hidden within the regiment.
Just some thoughts.