While I agree with Alcie, that more than likely you were diagnosed correctly 15 years ago (and 15 years isn't all that long to be on PPI's in the realm of things...many of us who have GERD have used them a long time). A endoscopy can diagnose reflux by identifying changes (often precancerous) on the tissue lining the esophagus. Your LES (Lower Esophageal Sphincter) is the culprit in GERD. Your stomach naturally has acid--it's the ability of the LES to hold it in the stomach that divides those of us with and without GERD. From all my research, stress creates more, not less, acid production, but I trust that you researched that fact thoroughly.
Many who have GERD have tried other things to solve their problem, including HCl. HCl is a very powerful acid, that can cause many problems itself. Be extremely careful if you try it. Have you discussed this with your physician?
As far as getting off Nexium. I discussed getting off Protonix with my GI doc, and he recommended the following:
Take it every other day for about a month, then stop.
Tapering is better, since you can get an acid rebound effect if you stop cold turkey.
Remember...if you have a less than effective LES, any additional acid you introduce into your system is likely to reflux into your esophagus and cause burning, which can create pre-cancerous changes in the lining of your esophagus.
Best of luck finding the solution to your problems!