I think it's hard to suggest directly that they'd lost they're taste for action but there are all sorts of signs on the part of individuals that something was wrong. You have: Surtees most directly and explicitly talking about some kind of mental collapse after Badajoz (he also suggests some soldiers committed suicide afterwards); Costello talks about soldiers deliberately exposing themselves to enemy fire to end it all, also about various crimes by the troops in that later period, eg murder; Kincaid in commenting about a small action after Vitoria makes his remark about having survived a great day, feeling he had the right to tell people about it; Simmons, having stayed with his battalion for years, tries to come home on leave during the winter of 1812/13, and ends up spending a couple of months out of the line nursing Col Barnard; during the Waterloo campaign you have examples given by Simmons of a long serving NCO who wouldn't advance when ordered to; Harry Smith writes about his difficulties relaxing himself when the campaigns ended in 1814. There are many other indicators we could talk about - eg around one dozen long serving NCOs being transferred to a veteran battalion during the winter of 1812-13.
Now of course some of these are not proof positive of combat stress, since I think with some of these older soldiers they reached a point through wounds or lameness where they felt they just couldn't keep up with the marching anymore. But my general feeling is of men who have increasing difficulties coping with the life, drink more as a consequence and become less disciplined. Did this cause a loss of performance ? The surprise of the 10th December 1813 is a most interesting example. It may be evidence of some other factors too - eg soldiers on both sides knowing the war would soon be over and not wanting to be the last one to be killed. All I can say - through personal experience as well as watching it in others - is that any man can reach a point where he has had sufficient brushes with death or injury to begin thinking he should be more cautious than he once was.