What do I do about pain?

I am a cart collector at a supermarket and was just called in to my bosses office yesterday and was told I was working too slow but the thing during my job I walk 10-15 miles a day which causes my feet, knees, hips, and back to be in a lot of pain and it really slows me down. What can I do to work faster through my pain? I have tried everything over the counter for my pain and nothing is working. I went to the doctor and because there is no injuries they just tell me to take advil or aleve. I currently use custom made orthotodics, good work shoes, and knee braces but they don't do anything to help. what should I do.