You really should go have your feet looked at by a certified Orthotist or Pedorthist. In general the more expensive shoe inserts tha are sold "over the counter", I.e. the Dr Scholls where you stick you foot on their machine are really the same thing as the cheaper gel pads and inserts.
Whoever you go to should be able to determine the source of your problem. It sounds like you have a hard heel strike thereby causing the heel of your shoe to wear out, that hard step could also he causing your pain. It could be solved possibly by putting a heel cup in your shoe. They are inexpensive and the professional you go to will be able to advise you on the exact type to get.
