Perhaps you are looking at the wrong end of the spectrum? Officers memoires (especially senior officers) are probably the worst source to consult on combat fatigue.
I'd suggest reading Jakob Walter's touching account of his experience as a Napoleonic foot soldier. Nadezhda Durova used almost the same words, that when one sees so much death, one becomes numbed to it. Even Parquin, a true "Chauvanist" speaks to the a weariness.
What does Rifleman Dodd's accounts say? (Haven't read it myself, but is handed out to the Marine Corps recruits at Parris Island to read)
As later postings note, anyone who's experienced combat (even modern combat) can speak to the mental and physical fatigue, compounded with boredom, anxiety between firefights, and the almost total inability to sleep due to the adrenalin pumping.
That senior officers did or didn't experience this probably relates directly to how far they came up the ranks and how close to death they were placed...