Hello,
Since the rash is confined to this particular area for so many years, so it looks like contact dermatitis to me. You can apply some calamine lotion at the rash as it will help in soothing the skin. You can take some OTC oral antihistamine medications like Benadryl or Claritin and see if it helps.
Also use mild steroid cream and good quality moisturizers to prevent moisture loss from the skin.
If still the symptoms do not improve then please get a clinical examination done by a dermatologist. It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
It is possible you are allergic to the elastic around your underwear, or the elastic is causing you to sweat under the band and promoting a rash.
Just a thought... since you say that's the only place it is.