How do I increase my troop stamina?

Troop stamina is not a thing. It's your lord stamina that's fading away. Consider your lord being a very old weak man and everything he does makes him tired. You see at your picture of your lord a stamina bar. That needs to be 500 if full. You also see a counter that say's something like 1/49 sec what means you get 1 point of stamina back per 49 seconds (in my case) you can improve that counter slightly by doing some settings mentioned in this same thread. Search this site with ctrl-f and use "stamina"

You find a bit more info there.

Remember this game is build around making you pull out hairs and grab your wallet to put your gaming mind at ease. Actually I'm wondering if you continued playing this game since it's been 11 months since you asked this question. If not, I do congratulate you for leaving the game.