I was on prilosec for almost a year. When I went off it I almost went back on. Your stomach needs acid and it's a good thing your trying to get off. It is a difficult process and you'll suffer for a bit but your body will even out. Do it very slowly and that should help a bit. Good luck!
Call the doc's office and get a refill. Then when you're stabilized, come off very, very slowly. Coming off of PPIs all at once will cause a rebound effect in many people. If you've been on Nexium for years, you should probably think of coming off of the med over a period of many months. Don't try to do it in a short period of time.