And you've never asked your doctor or GYN about this?
It is WAY beyond time you do, doll.
Could be an allergic reaction, could be a yeast infection that is now chronic from lack of treatment, could be a few other things.
Make an appointment with your GYN or local sexual health clinic, and for future reference, never let symptoms go on for so long without having a doctor look at them. I'd advise anyone with something like this to have it looked at after a few WEEKS. Certainly not years.