I suppose this is pretty broad since war has been throughout time, but it is something I don't feel like I've ever heard about and seen represented in media depicting war in any kind of "medieval" like scenarios (hand to hand/armies on a field), or even early gun combat situations.
We seem to get this question on AskHistorians pretty much every other week (and today, twice in the span of four hours). The FAQ has a whole separate section on it. Here are some useful older threads:
Was PTSD any more, or any less, prevalent amongst ancient and medieval soldiers than todays soldiers? - mostly regarding Ancient Rome
Are there any indications of combat PTSD in societies like the Spartans, Khan's or Alexander's armies? - Rome again
Did people in ancient/medieval times suffer from PTSD? - Classical Greece and the Assyrians
Can anyone here recommend any works upon mental health throughout history, specifically concepts such as PTSD, depression etc. and especially as a consequence of war/conflict? - in which /u/mythoplokos and I discuss the applicability of the concept to Ancient Greece
I hope this helps!
I can only speak from a Scandinavian perspective, but it has been put forth as one explanation of why early modern Swedish soldiers where significantly overrepresented in court cases concerning violence and deadly violence against civilians (and other members of the military). There are many examples of soldiers reacting with furious anger to innocent actions from civilians, like children throwing snowballs at them etcetera.
Suffering from traumatic experiences, paired with the fact that they were young men, trained to fight, always armed with swords and put in a situation where they repeatedly came into conflict with the peasantry made for a very unfortunate mix of circumstances. I actually haven't read any studies looking a the suicide rate of soldiers, but I would be surprised if no one have looked at it before.