Hello. I Am A 68 Year Old Active Female. I

Hello. I am a 68 year old active female. I have a tightness in my sternum area and it feels as though I have on a very tight bra. I have been tested extensively for any cardiac problems and was told there are absolutely none. I have been diagnosed with costocondritis. I have read that it should last from months to as long as a year, but my syntoms have exceeded three years. I have seen numerous gastro, cardiac, neurologists and cardiologists. No one knows why I have this tightness and pressure in my chest area. What could it possibly be.