Is the pain in your finger tips?
The first thing that springs to mind is to get used to playing longer. Much like running you need to build up stamina. It's good that you practice every day, but 15 minutes a day really isn't very long... it takes more than thin callouses to deaden the nerve endings. It took ages for pain to subside for me, and I was practicing several hours a day. At 15 minutes your fingers would be just beginning to warm up!
I have heard that rubbing alcohol helps growth of callouses (don't play immediately after applying) and Witch Hazel is good for reducing inflammation