30Guy,
You’re right about stress being a source of digestive problems. If you have another gastroscopy and don't get a definitive answer then I wouldn't have any more. Your doctor may not be able to help you. You might try another doctor but that may or may not help. If you're in the US there are a couple of things to keep in mind about the medical profession. Doctors love to do tests and procedures on patients with insurance, it makes them lots of money. Secondly, be very weary of medications they might want to prescribe to you. It is likely that your doctor is being rewarded by a pharmaceutical company for prescribing its products. I'll say a little more about drugs latter on.
DIET:
Pay attention to what you eat. Read the ingredients label. Try to avoid foods that have been heavily processed, especially refined carbohydrates. Stuff like sugar, enriched white flour, and corn syrups can wreak havoc on your stomach and intestines, not to mention your overall health. Look for ingredient lists that are simple and easy to understand. Products that have whole grains, unprocessed oils, regular salts and seasonings are usually best. Also look to keep some fiber in your diet as well. Avoid artificial sweeteners, hydrogenated oils, MSG, food colorings, and anything with a high sugar content. I find a good rule of thumb is to not touch anything with a very extensive ingredients list. If it takes over 50 words to describe what’s in a food product then it's probably junk.
DRUGS:
Try to avoid pharmaceuticals. Most of the new drugs that are now available have minimal or no effect for a lot of negative consequences. I'm thinking mainly about toxicity here. I'm not talking about something like morphine, a well known pain-killer that KingdomOfRats mentions above, but this new wave of "wonder drugs" that can supposedly cure anything. In most cases people are better off trying to cope with their ailment by adjusting something else or just putting up with it. I think you really need to get significant benefit from taking a prescription drug to justify its costs, both in terms of money and health.
Hope this helps.