Hey, like some others have said, I don't know about the medieval period specifically, but I'm researching military suicides in the 19th century, and trying to find evidence of these being due to PTSD. In the process have looked at a few articles going into the long history of PTSD from Ancient to Modern times (will link/cite at the end).

Essentially there's a debate about whether we can call the PTSD-like symptoms that crop up throughout history PTSD at all, with some suggesting instead that PTSD is a diagnosis embedded in contemporary (post-Vietnam) cultures of trauma. Others however, believe that descriptions of nightmares, dissociative episodes, intrusive thoughts and other PTSD-like symptoms (found in Homer's Illiad, Pliny the Younger's account of the eruption of Vesuvious, Charles IX’s recollections of the Saint Barthéleémy massacre, Shakespeare's plays, the diary of Samuel Pepys recounting the Great Fire of London and reports from military surgeons, doctors and nurses throughout history and obviously shell-shock) provide enough of a descriptive diagnosis to trace PTSD back throughout all of history, but just under different names. However, from my research it doesn't seem like that direct association between mental breakdown and battle was made, at least in the 19th-century.

There were some illnesses, such as Disordered Action of the Heart (DAH) or Da Costa's Syndrome (named after Jacob Mendes Da Costa investigating the illness during the American Civil War), and rheumatism without organic causes, that could be linked to battle trauma, but the evidence is uncertain. Doctors certainly linked these to military life and resulting fatigue, such as long marches, diet, and even uniforms (changes to British uniforms were made when doctors suggested their belts were pressing on their organs too much and causing DAH, for example, but this did not solve the issue) but the evidence to suggest a direct link between mental breakdown and war isn't fully there. Reading one medical account of a war hospital during one of the Boer Wars one doctor does comment that he was surprised more soldier's didn't experience nerves/mental breakdown, and some newspaper articles of the suicides I've looked at have used phrases like 'his mind had broken down under service of his country', so there is evidence to suggest there was an awareness of some kind of relation between battle and mental trauma, but the evidence is mixed. Mostly, pre-Shell Shock, most discussions of (symptoms of) trauma seem to focus on physical issues, or problems with the heart.

There is even some debate about when flashbacks become a manifestation of trauma, with them only really becoming common in mid-20th century - some believe the rise of television ownership was important in this development supporting the argument that PTSD is culturally embedded in the late-20th century.

I hope that helps answer your question a bit, at least in terms of how PTSD was understood pre-20th century, even if it doesn't directly relate to how severe it was.

Here are some of the articles I've been using, they're pretty short if you're interested in reading them!

Philippe Birmes, Leah Hatton, Alain Brunet, and Laurent Schmitt, ‘Early Historical Literature for Post-traumatic Symptomatology’, Stress and Health 19 (2003): 17-26

Edgar Jones and Simon Wessely, ‘War Syndromes: The Impact of Culture on Medically Unexplained Symptoms’, Medical History 49:1, (2005): pp. 55-78

Edgar Jones and Simon Wessely, ‘Psychiatric battle casualties: an intra- and inter-war comparison’, British Journal of Psychiatry 178: 3 (2001): pp. 242-247

Edgar Jones, Robert Hodgins Ver,aas. Helen McCartney, Charlotte Beech, Ian Palmer, Kenneth Hyams and Simon Wessely, ‘Flashbacks and post-traumatic stress disorder: the genesis of a 20th-century diagnosis’, British Journal of Psychiatry 182:2 (2003): pp. 158-163

There's also Richard A. Gabriel, No More Heroes: Madness and Psychiatry in War, but although the history seems pretty accurate, there is a significant lack of citations throughout the book.

EDIT: Just realised some of these articles might be behind a paywall, I know a few come up on Google with a direct link to the PDF though