Bob Cole is scratching the surface of it here.
I used to have the exact same problem plus I would actually get a backache from the tension.
I finally figured it out and this is probably going to sound really stupid but it worked for me.
What I was doing was operating a machine that delivered bullets into a target.
I was holding it in a white knuckled death grip and putting WAY too much emphasis on getting the sights lined up perfectly and concentrating on getting this rock steady stance so I could deal with the recoil and get back on target fast.
It was wiping me out mentally and physically after just a couple boxes of .45ACP.
What finally turned it around for me was this,
I quit operating a machine and became the machine itself!
I met up with this karate kid who broke bricks and stuff and he told me that he mentally put his fist on the other side of the brick and just did it. everything was just one big flow with him.
Then I saw an interview with some race car driver who basically said the same thing.
He said his whole game was played way out there in front of the car
So I tried it myself.
I took all the moves. Setting my feet and shoulders, sweeping the cover garment away, gripping the gun, drawing the gun, getting the front sight on the bull, hitting the safety, bringing the rear sight up forming my sight picture and pressing the trigger.
Then I took all those seperate moves and glommed them all into one smooth movement while focusing my mental game 21 feet away at the target.
As stupid as this may sound I started thinking the bullets into the bulleye and it worked!
Pretty soon I was (rather slowly) putting everything into the bull with NO fatique whatsoever!
Then I started getting faster and the groups shrank down to hand size, then fist size.
Then I realized I was going through 5 or 6 boxes without getting tensed up.
Thats pretty much where I'm at right now but I'm sure I could go through a case+ without getting all tightened up.
This may not work for you but it did for me.
Sometimes I actually feel like I come off the line refreshed, looser and with more energy than I had when I started.
Like they say however "Your mileage may vary!"