Nausea without reflux sounds like gastrits to me. It would be worth continuing the Zantac even though you don't notice improvement, as it can protect the stomach from some of the acid onslaught. I have a very reactive stomach and get gastritis fairly often. Whenever I do, I take acid suppressors to help calm things down. There is one thing that really helps me a lot...Carafate suspension. It does a great job of coating your esophagus and stomach and helps provide comfort and allow healing. I'm never without it in the house. If you have not reflux and all the acid is doing its damage in the stomach, then you could take the generic version, Sulcrafate tablets. These are both prescription drugs, but very safe.
The other thing is WHAT you're putting into your stomach. Imagine the inside of your stomach like a raw sore. During the digestion process it contracts and mashes food. If there is anything in there that is rough, it can cause even more irritation. Only put things that are fully chewed to a smooth consistency into your stomach. If you eat softer, smoother foods, you'll give your stomach a better chance to heal. Obviously, stay away from acidic and spicy foods, and alcohol.
My GI doc told me that when healing a gastritis I should eat carefully until two weeks after my stomach feels perfect. I have found this to be good advice. The temptation is to "cheat" once you're feeling better, but if you do, you're almost certain to get worse again.
Good luck with your endoscopy. I'm sure it will provide lots of answers.