Okay this sounds familiar.
First I am no bootfitter, but I have been skiing for decades....so I understand your pain.
Listen when you foot is not supported properly underfoot it goes flat; pushing the arch and ball of your feet down forces your foot to expand outwards, effectively pressing the sides of your foot into the shell of the boot.
Check out your insole. If it is stock, it most likely has very little support structure. Your foot isn' t flat so why should you insole be?
Go see your friendly neighborhood bootfitter he will look at your feet and assess whether or not you need custom insole or simply aftermarket ones. If that doesn' t work the bootfitter might decide to punch out your boot a little for relief.
Make friends with a bootfitter, it is worth every penny.