I think your mind is your own worst enemy when shooting/hunting.
I'll be the first person to admit it. My breath gets short, wrist and fingers get twitchy and my knees get a bit shakey. But...when I start to draw my bow and pick a spot on the animal, I tune everything out. Its hard to explain, but my mind blanks out and for a few seconds, I can't remember anything. I don't even concentrate that hard on my midterms! (but nothing dies during a midterm, except maybe my grades..)
But during a 3D shoot...anything goes. If I'm keeping my own score, I generally fall apart. If I make it to a shootout and have 1000 people watching me, I fall apart. But, if one of the other guys in the group is keeping score, and we are heckling each other, and all is good, there's a chance I could win something.
On a side note, if I'm stalking an animal, I don't feel the least bit of "buck fever". But if the animal is coming towards me while I'm in a blind or in a stand, I get as jittery as anything.
Its interesting how diffrent situations have their own type of stressors.