It is difficult to say, but let me give you some general historical thoughts and then some ideas based on what I've read about veterans. Identifying "psychological disorders" as we understand them today (and of course we do not fully understand any of them even now) is largely an unrealistic project. Generally, even social historians have hardly considered people with any disabilities in their work. Since people with disabilities were not a major field of study until recent times, we would have to delve into the "hidden archive" of what surviving texts suggest or allude to. It all becomes quite speculative.

We also have to consider the shifting definition of "psychological disorder." Not to get too postmodern here, but what we consider to be a disorder has changed frequently over the past century. The go-to example would be considering homosexuality to be a mental illness by the APA and WHO (also some people still holding on to the idea that it's something that can be cured with a camp). But think about nuanced are anxiety disorder or bipolar disorder. We can probably identify cases in well-document people, but to draw general statistics from "exceptional" people is a speculative task. The closer to today you go in history, the more documents we have in general. So, tracking disability in Vietnam veterans is easier than the same in Civil War veterans, but Civil War vets are easier than Revolutionary War vets. You can look at pensions, riots, rates of employment, ages of death, suicide, and the occasional personal document (diary, letter) to extrapolate information about PTSD and other disorders in the general veteran population.

I read this blog post recently on Nurse Clio about veterans with disabilities after the American Civil War. You can see that bringing up disabilities can create a historiographic battle. I believe people researching and publishing on the physical and psychological affects of war on veterans are doing everyone a great service. Even Gaines Foster discusses disability among Confederate veterans in Ghosts of the Confederacy (1987). It's an important piece in understanding the "Lost Cause" and other reasons the Civil War continues as an intellectual war even 150 years after the fighting ceased.

Another interesting study is Jonathan Shay's Achilles in Vietnam and Odysseus in America, which compare the experiences of Vietnam veterans with PTSD to characters in the Iliad and the Odyssey. It's really brilliant work that demonstrates the possibility that we can recognize psychological injury in ancient texts.