Good job quitting the cigarettes!
Sometimes, our instincts tell us something is wrong when the pros are telling us everything is fine. You need to have enough information from your doctors to be confident that you are healthy. That you are experiencing pain should prompt your doctors to take a closer look. If you haven't done so already, you could get an opinion from a National Cancer Institute recognized Comprehensive Cancer Center. They are the experts who would be best equipped to tell you if you have something that is malignant or if it is normal. There is a list of NCI CCC's on this website.
Other than being told it was normal, have any of your doctors given you an explanation for your tongue being persistently white, and now, a bit painful? Are you taking any supplements or eating/drinking something that could be causing it? Very strange that it isn't going away.
You had a throat culture for strep and mono that were negative. Did they take a culture of your tongue?
