This will sound silly but for years I had a sticker on my upper limb (I think PSE was distributing them) that said "Stay Calm, Pick a Spot" .
It was right on the face of my upper limb staring at me anytime I had my bow in my hands.........you couldn't help but read the words.
Blanking out and shooting at the whole animal is a real problem when you are nervous, and a reminder to relax and AIM is a help.

The nerves will settle down with experience........I remember some of my first encounters wondering if I was having a heart attack.[:-]

For a while, force yourself to concentrate on the mechanics of the shot.......as soon as you know you are going to get a shot don't come unglued, start to immediately think about how and where you will place that arrow.
Stop the animal if necessary with a soft mouth bleat and bear down.

Also. Do not blank out AFTER the shot! Keep it under control long enough to understand where the animal was hit (if possible) and watch it carefully as it runs out of sight. How it ran, body language and carefully mark in your head an easy landmark where you last saw the animal.......something you can find from the ground.
What happens AFTER the shot and your nerves is every bit as important as what happens BEFORE the shot to help in a successful recovery.