I'm really surprised with this going on as long as it has, that you haven't been referred to a Gastroenterologist.
That would be my advice: seek a GI specialist's advice.
It could be stress - it's a really powerful force. It could be psychological. It could be physical and needing different medical attention.
Prevacid is a good Proton Pump Inhibitor, but it doesn't mean it will work well for everyone. There are many others on the market (Prilosec, Nexium, AcipHex, Kapidex, Zegerid).
Regardless, the nausea/vomiting are not common symptoms, and I think they warrant another, more specialized opinion.
Good luck.