you're one of the person who can't stand playing FPS too long.
Some of my friends has this symptoms. It's a minor epilepsi some of them said. You can't cure em, but definitely take a break when you started to get dizzy or nauseus.
Some solutions including third person view, so the player can focus on the player model instead.
I've been playing FPS games since Wolfenstein so this kind of game never been a problem for me.