I was reading about the hundred years war and obviously with combat being what it was back then some of the accounts of battle are quite horrific. Specifically what comes to mind is when longbows were used to great success by the English. Repeated charges up hills for days at a time only to be shot down or have to engage in extremely violent physical combat. This must have been extremely stressful mentally. We know that PTSD has been called different things throughout history but I've never heard if mental health of medieval soldiers was recorded in historical texts or talked about much at all.
Interest in PTSD and its historical antecedents is currently strong, and this is a topic that comes up on AH fairly frequently – so much so that there is now a section of our FAQ that overs links to earlier discussions of this problem.
There is always more to say, of course, but while you are waiting for fresh responses to your query, you might like to review some of these responses.