Honestly, it sounds like your shell is simply too big and now that the liner has settled, it is too sloppy.
Do this: remove the liner from the shell, step into the shell with your ski sock on, slide your toes to the front of the boot, and see how much room is behind your heel. 1cm = race fit, 2cm = normal, 3cm = too big and problematic.
If you have a 3cm shell fit, then your boots are the wrong size and the only real solution is buy the boot that fits. Any attempt to make the slop go away will only be temporary since the shell is wrong for your foot.