Is there any tips on how to stop my shoulder going out when holdng unsupported?

I'm going to put adaptions in anyway as I've got a spinal injury but currently I shoot on my garden range with my back supported upright, right hand on the trigger and left on the foregrip and I keep finding after 20-40 shots my left shoulder gives out, doesn't seem to be related to Rifle weight either as it happens as much with the Daisy that weighs nothing as it does with something like my BSA or Webley, I'm also a big guy to boot.

Thanks in advance for any advice or tips.