I tell myself...."aim small, miss small"....then I yank the hell out of my trigger while ****ting myself. Im trying to calm down but it's hard and I put so much heart and soul into this.
Bear44 is right about the trigger making it easier. Would probably find you shoot better under stress with something that kicks less as well, since all the factors get amplified.
The nervous feeling when you see a deer is adrenaline. The next time you see a deer and start to get nervous do this: take as big of a breath as you can while regaining focus and composure. Exhale slowly and remain focused. Continue breathing deeply and slowly and exhaling fully and slowly while observing the deer and what it's doing. It will never know you're there. Your goal is to remain calm and ready to act.
The thing about adrenaline is that it escalates much faster than it dissipates, so the faster you get control of it, the less it affects you, but if you let it get too high, it will absolutely work against you for an extended amount of time. It's very hard to bring back down once it's elevated so controlling your composure and breathing, in the beginning, is essential.
Too much adrenaline destroys your fine motor skills, seen when you jerk the trigger instead of slowly squeezing the trigger like you would at the shooting range. Start paying attention to it while you're in the stand and you'll learn how to control it.