When I started rifle hunting for deer, I wasn't a great shot. I never missed, but I always noticed the bullet hole wasn't where I had aimed. So I spent a day with a bag of targets in my clear-cut. I shot my hand-loads all day (my best work, they were extremely uniformed loads) and discovered just how inconsistent I was in my aim.
I worked on my breathing and heart rate to no avail. I put the rifles down for a while and started plinking around with the 22-250 and the .223
I noticed I was starting to get a little pudgy, guess my metabolism started to slow down, so I started running regularly. I am up to about 20 miles a week and feeling great. I dropped my heart rate down to 48 resting and there's a huge difference maker right there. I took the rifles back out a few days ago for the first time (I had shot a couple times between then and now, but nothing serious) and I was steady as a rock. I could slow my breathing down immensely, which made me super-steady. My heart rate was nice and slow, and I was the best shot I've ever been. I was nailing pie-plates every shot at 400 yards. I know, that's nothing special to many of you, but for me, that's huge!
My problems all stemmed from breath control. I don't recoil-flinch anymore because I suppose putting several thousand rounds out my 12 gauge every year makes me pretty immune to recoil. To eliminate this possibility, I did the dud test. I had my brother load 4 live rifle rounds in the gun, and one snap-cap. He loaded them randomly, so I didn't know when the cap was coming. If I was flinching, but we couldn't see it on live shots, we were going to see it on a snap-cap. So when the snap cap came up, sure enough, I was steady. So that was ruled out. I suggest that test to anyone who thinks they have a flinching problem but can't really prove it.