My vigina Burns!!!!

Hi I have been having sex for about 6 years now with the same guy, who is now my husband. I do not have any stds or any problems like that, however I have been have this horrible burning for 6 years on and off. I have been tested for everything. Its not a yeast infection, bladder infection. I have been to over a couple douzen doctors and no one can help me. This is very frustrating to me, that no one can tell me what what it is. I have tried everything. Not having sex, not using condoms, more lube, less lube, no more tampons, EVERYTHING and nothing works. If feels like a bladder infection but all the doctors say I tested Negative for it. It come and goes, but I always have discomfort. And when it comes, it burns so bad that I am unable to move, besides to go to the bathroom. The burning starts on the outer side of my vagina, and burns all the way up. Like a tube that is on fire. I take AZO to help with the pain, but it can only do so much. I do not have health insurance anymore HELP