Suggest Treatment For Skin Darkening And Dryness Of Feet

The skin on the heels of my feet thicken and turn gray...making them appear to be dirty. They also stay dry, no matter what is used on them. When they get wet, the skin turns white and puffs up. I was prescribed Umecta, to apply twice a day. After almost 2 years, there is no change or improvements. Currently I soak my feet and remove dead dry skin weekly. It only takes a matter of days before the skin returns dark again. This has been an issue for the last 5 years. Today, I hide my feet constantly as I can't seem to nip this problem, and am unsure how it even started. Please advise.