Cracked heels that won't soften no matter what I try

My heels have been cracked and rough for months now and I am running out of ideas. I file them, I use a foot cream, and within a couple of days they are back to being hard and a bit split at the edges.

Is there a proper order to do things in? And is there a particular kind of cream that actually works, or am I wasting my money? Any help appreciated, my feet are not happy.