If you want to ski at a high performance level, which park skiing generally is, you need a high performance fit.
When you get shell fit for your ski boots you don't want more than a finger widths of space between you and your shell. If you are just skiing groomers a couple times a year then your average REI boot fitter is just going to throw you into a rec fit boot which means 2 fingers plus in a shell fit. This sort of fit means extra play in your boots, which makes them comfy, but also means your foot can just squash the foam of your liners down and slam around inside your shell leading to boot bang, shin bang, toe bang and another kind you can think of.
You probably need a better fitting boot, but things like custom foot beds, adhesive foam or shims and booster straps can help in the meantime. Best bet is to see a good boot fitter as you need someone who can diagnose your problem in person.