I am a little surprised that anyone should be surprised that combat fatigue should be found in the Napoleonic period. "Shell shock" became quite common in WWI, which was also the first time that such a large proportion of soldiers became exposed to such massive firepower over prolonged periods. However "shell shock" and "combat fatigue" are not the same thing, thus combat fatigue is not restricted to post 1914 wars, though one could argue that shell shock is. There is a lot of research ongoing about a whole range of related mental effects under the umbrella of "post traumatic shock disorder". People subjected to sudden shock OR to prolonged stress can exhibit clear indications of personality change. People subjected to both are at heightened risk. This is not restricted to the military. Abused children and coal miners exhibit many of the same symptoms, to name but two groups.
Imagine a soldier who spent a number of YEARS on active service. During that time he would have marched hither and thither with often no knowledge of what he was doing or why, at a time when commissary and medical services were scant. So on any given march every soldier had the risk of death by starvation or illness. Sound extreme? Look at nearly any memoire for casual mention of such. While the officers had better knowledge of what was going on and better resources for food and medicine, stress is cumulative. That Picton by 1815 was feeling done is no surprise.
Add in the certainty of some kind of fighting along the way and stress goes way up.
As an aside, there is a major difference in how soldiers from modern, and in particular, western armies are likely to die compared to pre-1914 wars. With current logistical and medical support systems, soldiers no longer expect to die of disease or starvation, though it does happen. In the Napoleonic era, a soldier was far less likely to die in combat than from any other cause.
The relentless and ever present prospect of death, coupled with the inability to control one's own fate, leads to a persistent level of stress. Periods of inactivity help to reduce that stress. Individual soldiers, and units, must learn to operate with an elevated level of stress. When they do, they become relatively robust and resilient. However, there are limits to what individuals can absorb and by extension to what a unit of such individuals can absorb. As those limits are crossed the individuals become unpredictable and the units become brittle. Chances are that some of you have seen this in non-military environments with individuals having "mental breakdowns" and teams imploding.
As for the "thousand yard stare", there are two sides to that. When first exposed to a heightened level of threat, most people exhibit a degree of confusion. Once that confusion has been overcome, they are able to process the incoming information and act accordingly. Such a person is operating very efficiently, but is also very focused. Given that miscalculation can lead to death, all incoming information is sorted by important or not and all mental effort is placed on what is important at the time. By analogy, athletes can develop a sense of "being in the zone" and ordinary people in a life threatening situation will often speak of time dilating as they sought to save themselves. Churchill once wrote that "nothing clarifies the mind so well as being shot at". In the first stage of the "thousand yard stare" the soldier is effective and focused. In the second stage he is exhausted and still focused on the threat, but less and less able to cope.
Given the level of stress for a Napoleonic period soldier, I cannot see why individuals would not suffer from stress induced mental illness.
Rory mentioned looking for examples of long serving units showing signs of exhaustion. I would suggest that the length of service would need to be cross referenced with the policies on replacement. For example, by the time of Eylau, there were indications in Napoleon's army of unit level fatigue. Many of these units had been marching and fighting with no rest in a challenging climate for months. Even with some replacement coming forward, the bulk of men in the ranks would have been through the entire campaign to date. So in 1813-14, how much of a long service British regiment had been in the ranks for years, and what proportion had just arrived?
The Peninsula may be very instructive as a long running theatre. One must note that between campaigns, there were long periods of refitting. Even on campaign Wellington was able to take advantage of shorter land communications and better logistics. A prime example is the fattening of his army behind Torres Verdras while Massena starved.
As for Massena not having the same fire in 1809, well he was getting a bit older. How many of us in our forties and fifties are as aggressive as we were in our thirties? And when we are aggressive, can we keep it up as long? I personally find a lot of truth in the adage that old age and cunning will overcome youth and enthusiasm.
And as for certain senior officers being accused of preferring their luxuries to the rigours of the field, well, how could anyone be surprised? When a middle aged man, with a familiy of young kids, has risked his life countless times and been rewarded with a comfortable house, good income and the time to enjoy his family I think it entirely reasonable that he should wish to enjoy it and let some other young buck make his mark. When such men still put aside their comforts and took on the risk again, it is admirable.
One can surely find parallels through all of written history in all cultures.