metr0Could it just be my boot that's too stiff? Because I got it fitted and everything worked well. I'm 6 feet tall, I weight around 140 lbs and my boot has a 120 flex.
99% no. Impossible to know for sure without seeing you in the boot, which is why a boot-fitter is so important to actually solving this problem. It's like having a tooth ache asking the internet why your teeth hurt.
1. What is the shell fit of your boot like? Remove the liner from the shell, step into the shell bare footed, slide your toes to the front so they just barely touch, examine the space behind your heel. 1cm = high performance fit, 2cm = normal, 3cm = problematically too big
2. Do you have custom footbeds? If yes, skip to #3. If no, then this is an absolute must for you and without them any solution you try will most likely not work since this is so so so so fundamental in securing your foot and preventing your shin from literally twisting in the boot.
3. Do you have lots of slop in the fit of the cuff of the boot? You ideally want a snug, firm handshake fit (in general) but definitely in the cuff too. No gaps between your shin and the tongue of the liner. A gap behind your calf is ok as long as there is no gap up front.
4. Are you landing backseat a lot. A good boot-fit can be ruined by poor skiing or mistakes while skiing.