You should have listened to your boot fitter... without a custom footbed, your foot (which is more flexible than the boot) will twist, move, shift, slide, expand, etc. inside the boot. A custom footbed creates the interface between your flexible foot and the rigid ski boot, holding your foot in the proper place.
When you get a custom footbed, you will need to have your liners remolded because your ankle will be in a different position. If you don't do that, your ankle & heel won't be secure either.
All of this presumes you are in the right size/volume boot for the size and shape of your foot. If you simply picked a boot out because it was what YOU wanted and not what the boot fitter recommended, then you are doomed from the start.
Order of operations:
1. be in the correct size & shape boot for your foot.
2. be in the correct flex for your ankle mobility, height, weight, strength, etc. (not if you ski park or all mountain).
3. have a custom footbed that properly supports your foot.
4. have the liner molded.