Ben what you are experiencing is totally normal and I think everybody has that at first, I certainly did. And don't worry, even after hunting for over 20 years and shooting 3 deer every year, when that big boy shows up with a rack that will fit a case of beer inside it, you will feel that buck fever all over again!!! It's all part of hunting, mostly it's a matter of just realizing that you are going to feel that way, and getting used to it. Also I have found that sometimes you just don't have time to get nervous, things happen too fast. My blood pressure still kicks up for the most part, but I expect it.
Some tricks that I have done to help:
1) When you practice, that first practice shot is the one that counts. Make sure that you pretend that you see that buck out there, imagine him walking by, you get your shot and take it! During the season, I try and practice every day - just 1 shot only. (and more practicing from time to time)
2) This will sound silly, but you can get your gear on, run around to get your blood pressure up high, and then take your practice shot. This will get you used to shooting well when your adrenalin is going full bore.
3) Maybe in your case concentrate on a 2 second count after you release your arrow. "Still as a statue" or something like that. Do that enough and you will have muscle memory like the athletes use, everything will happen all by itself then.
4) When you are up in your stand, plan out how exactly you will shoot a deer depending on the angle of his path. You know, at this tree I will pull back, at that shooting lane is my shot. Then when the real thing happens, you follow that plan. This also gives you something to do while waiting. When shooting at the deer, concentrate on a specific hair that you want to hit.
Mostly, you need to concentrate on the mechanics of the shot, you will have plenty of time to worry, get excited after the shot is made. Easier said than done!
Just some thoughts. It will get easier to deal with the more experience you get. But it is also what makes bowhunting exciting!