Ignore skia888, the spk will be 100% worse for your foot. It is a bucket compared to the Foil.
As a start I would ask, are the boots fitting alrite everywhere else?
If the answer is yes, then the solution is likely easy.
Yes : Go to the local shop and explain your problem, and that is a localised problem only in your heel. There are two things that can be done that will more than likely solve your problem.
The first are heel risers, these are pads about 10mm think that sit under your liner on the running board, these push your heel up in the bucket so you won't move as much, this may stop it all together depending on the rise you are getting.
The second can be done with or without heel risers, we call it the banana or horse shoe. It's a bit of foam that sticks on the outside of your liner in the heel area. The liner is heated up in the molding ovens then this foam piece is applied to the back of the liner (it basically goes from the right side of your heel, up like 10/15cms then arches over and comes back down the left of your heel.) then you put your liner in the boot, and your foot, then let it mold for 10minutes. These two things should solve all of your problems if it's just in the heel. It should cost a max of $50, and a lot less if you are in with the shop.
No : It may be more complicated but new boots may be the answer.
Get at me with some more info and i'll try and help some more but that should get you going.
Do not take boot recommendations over the internet.