So many have said their part, and everything I am about to say has already been said but I'll share my experience anyway. I shot a 270 win my whole life, got into big game and western hunting and wanted a bigger round so I bought a 300 win mag without a threaded barrel that would bruise my shoulder after shooting a box of ammo. Then I decided I wanted a muzzle brake and a heavier barrel for more accurate and to reduce the recoil so I bought a rifle with those features and I started jumping out of my seat every time I pulled the trigger. I could feel the blast from the brake hit me like a slap in the face, I would blink and pull my head back a little... after a few boxes of ammo this because anticipatory and I flinched bad - like shooting 7" groups at 100 yards kind of bad.
I consulted the internet and did all the drills people recommended like having someone randomly load or unload my rifle so I didn't know if I shooting a round or not... several 100 rounds of this stupidity later I gave up and started shooting my 300 win mag without the brake for some random reason, and after a box of ammo I was sub MOA and closing in on 0.5 MOA. Keep in mind, the rifle without the brake would literally jump after each shot and I'd have to reestablish my shooting posting each time I pulled the trigger, and despite this I was both precise and accurate.
It was at this point I realized my issue was not felt recoil but rather the blast from the brake. No fundamentals of shooting lecture will help you to get used to a constant high velocity cloud of hot gas and dust hitting your eyes - you are biologically programmed to blink and flinch when this happens. I doubled up my hearing protection, put on actual safety goggles so the blast stopped kicking dirt into my eyes, threw on a down vest and a carhart jacket overtop to reduce the concussion I felt, pulled my hat down over my eyes and scope, and pull the hood of my jacket over top of that... and I laid down prone behind my 300 win mag with the big brake and proceeded to shoot 0.5 MOA group after 0.5 MOA group. It was cathartic. I shot like this for several months just happy to shoot my rifle accurately.
... and then I bought a brake the didn't kick so much junk back at me, and its been fine since (even with standard ear/eye pro and a tee shirt). So the moral of this story is to buy more stuff. Its not you, its your gear. You just need more rifles and muzzle devices.