Some good points already mentioned. Shooting from a bench amplifies felt recoil. Most are somewhat scrunched down behind the gun. If you can get the gun high enough that your back is straight up it helps tremendously. he Brits used a standing rest to zero their heavy African guns, it allows you to roll with the recoil.
Traditional American stock design from the black powder era was also generally pretty poor. Too much drop, inclined combs, both magnify felt recoil and muzzle rise. Part of why I dropped out of shooting muzzle loaders for a long time was the horrible stocks most had, and I of course rather poo-pooed the stocks that "werent traditional". I eventually got over that, as well as realizing most English sporting stocks were far better designed than American ones, so i could somewhat accept better designed stocks as "traditional enough".
Lowering combs shouldnt be a stumbling block. As someone mentioned, motivated shooters grinding on high grade expensive guns to allow them to fit better, and some agonize over relatively cheap guns being modified to fit. Dad ground combs down on several high grade shotguns to fit him as well as adjusting length and balance as needed.
As a first step, reduce the load to a comfortable level, and work back up if you really need the extra power for hunting or longer range, and work on the shooting technique and stock as needed. Its probably not insurmountable.
Some comments reminded me of things Ive heard over the years such as "the trigger pull was too heavy so i sold it and got a _______(fill in blank) that had a better pull ". Uh, did you think to get the trigger pull corrected by someone?
