Are you flinching from pain or from the noise?

If it's the noise of the gun you have to mentaly overcome the flinch. Start with double hearing protection, foam ear plugs and a good set of ear muffs. Then have someone else load the gun randomly with cartridges so you can't tell when a loaded round will be under the hammer. Then consentrate on the sights and target and slowly pull the trigger. Keep you mind on the sights and try not to flinch just like the gun is not loaded. Hopeful in time you can work thru the flinch.

If it's from the gun recoiling and hurting you hand then you need to change the grips, the style of gun (ie, bisly grip instead of hog leg) or reduce the load, wear shooting gloves so you can handle the recoil without pain. You might also need to change the hold on the gun for more control. Then practice and consentrate on the sights.

Lots of heavy rounds in one range sesion can be counter productive as the body wears down. More range sessions and few rounds per session might help. Quality practice over quanity. I remeber reading that Elmer Kieth didn't shoot a large amount of heavy rounds a year.