It sounds like you're shooting from a bench on rested positions. That is the most "brutal" position to shoot from. The next is the "prone" position. Your body has to take up all of the recoil without being able to drain some of it off by being able to follow through as it pushes.
You don't mention which model of Mosin you're shooting. M91,91/30, M38 or M44.
If you'r shooting a M91 or M91/30, the sling helps, maybe. It only helps if you have it adjusted properly to suit your particular ergonomics but it is a pain in the butt as well as causes accuracy issues when shooting off the bench. It gets in the way and makes for an insecure rest on the front bag.
I find the first thing to tame the recoil on any of the M91, M91/30 and M38 models it to attach the bayonet. It adds weight to the rifle and settles down the harmonics. The rifles are sighted in from the factory with the bayonets fitted.
The next big thing most people do, especially when shooting from the bench is they don't hold the rifle properly. By that I mean they don't pull the rifle snugly against their shoulder when firing. This is understandable because IMHO most benches are to darn low to be conducive to comfortable shooting and I am not a tall man. Most shooters have to hunch over their rifles, which gives them and uncomfortable and unnatural position for aligning the sights to the target and is often a great way to develop a flinch.
The next thing is "proper trigger control." The MN rifles have two stage triggers. The proper way to use such a trigger is to take up the initial light pull and then slowly squeeze the heavy pull until the rifle fires. Many folks just pull the trigger all the way through both stages with one quick pull. This makes for a long/creepy pull that is long enough to allow a flinch between the beginning of the pull and the discharge such as a blink or tensing of muscles.
Now, I will admit I am very recoil sensitive. I need a shoulder replacement and just about any recoil at all is painful. This means I cheat. I don't shoot my milsurps anymore because of the recoil. I have attached muzzle brakes to my hunting rifles and when I do on the odd occasion shoot my milsurps, I use a PAST recoil pad. In some cases I use a lead sled. The sled works very well but it's a pain in the butt to haul the bags of lead to the bench from the truck and get set up.
Those fellows that you see shooting offhand in the videos do a lot of shooting. They should be able to hit targets offhand out to a couple of hundred meters without difficulty. If truth be told the same should be able to be said about anyone that hunts game animals. IMHO not nearly enough people know how to shoot or are able to shoot well from the standing position because they just don't practice enough.