Historian here, I wrote a paper last year on the health of Soldiers in the army of Napoleon. Now, I know this is not from the timeframe you're looking at, but maybe it helps.

Anyway, I studied psychological problems too because I thought it was interesting. In the time of Napoleon, there was this one disease in the army called "Nostalgia" (aka nostromania, philopatridomania, pathopatrialgia and nostrassia). Nostalgia literally means "homesick". It was the only psychological disorder in the military I could find by reading medical encyclopedia books and journals from that time. Two questions though.

1. Did Nostalgia, or homesick, have the same meaning it has today?
2. Can we link Nostalgia to PTSD or any other stress disorder?

I tell you, it is very difficult to answer these questions. I read the journals of some doctors, they basically all came to different conclusions about what Nostalgia actually was.

The most early source about Nostalgia is "Dissertatio medica de Nostalgia, oder Heimwehe" by the Swiss doctor Johannes Hofer, back in 1688. In France it was known as "Maladie du Pays". Now Hofer said he was the first to write about it, although we find some traces of the concept of Nostalgia in sources from the Thirty Years' War (1618-1648), where in the Spanish army it was known as "Mal de Corazón".

The symptoms Hofer described, to some extend, are a bit like the ones we understand today as symptoms of PTSD, but it's really difficult to completely understand what people thought Nostalgia actually was, and the simple reason is, they didn't really understand it themselves. For a lot of them it was just this strange disease, be it just psychological, or perhaps caused by a physical injury or trauma.

How was nostalgia treated? Well, it wasn't. If a soldier could no longer function, maybe, I say maybe, he was sent home. But what after that? Nothing, or at least, I couldn't find anything. The French historian Nathalie Petiteau claims a lot of the French soldiers came home with psychological traumas, because a lot of them committed suicide later.

I dedicated quite some pages in my paper to the subject, and my only conclusion was that not a lot of research has been done on the subject, and that an interdisciplinary research is needed to actually try to understand what nostalgia was. Was it PTSD? Did doctors understand what it was?

So yeah, I was pretty disapointed myself that I couldn't formulate good answers to the questions above (and your question), but that's just part of the job.

I hope more people will respond to your question, because I'm interested myself and want to see what others think!

Edit: Spelling and stuff. Sorry, English is not my first language.

Here is the account of a military doctor in the late 18th century. “When young men who are still growing are forced to enter military service and thus lose all hope of returning safe and sound to their beloved homeland, they become sad, taciturn, listless, solitary, musing, full of sighs and moans. Finally, these cease to pay attention and become indifferent to everything, which the maintenance of life requires of them..."

Josef Leopold Auenbrugger in his 1761 book Inventum Novem.

For more: http://militaryhistorynow.com/2012/09/17/walking-wounded-ptsd-from-ancient-greece-to-afghanistan/

Basically, PTSD has always been with us, even if we didn't always recognize it.

EDIT: A number of comments here have stated that the intensity of warfare was somehow magnified beyond previous combat in the modern era. Whether that's true or not is dubious and ambiguous. It certainly does not mean that PTSD is a creation of artillery and bullets and mustard gas and drone bombings. PTSD exists outside warfare (sexual assault, near death experiences, grievous injuries), and those have undoubtedly been a part of the human experience since its beginnings.

Not to discourage further discussion, but this topic is listed under the popular questions / FAQ. You should check there for some good answers on the subject.

You may be interested in the 'Post Traumatic Stress Disorder before the modern era' section of the Popular Questions pages.