I bought a 870 youth with the 21" barrel for my mother a while back for HD. It hurt her hand in the web. I shot it quite a bit to break it in and it did surprise me how much recoil it had compared to a 12ga 18.5". It has standard wood furniture. Is that the culprit. I was shooting mainly the remington 100/box of 7.5 shot.
Some of the problem I know is a weak grip. But she is 65.