Hello and thank you for posting your medical question on E health forum.
Based of the history provided, your symptoms of long standing abdominal pain could be either due to Gastro esophageal reflux disease or due to Irritable bowel syndrome. Pain due to gastritis of GERD is usually very severe and can mimic many other serious conditions including a myocardial infarction (heart attack).
Since an extensive battery of tests have been done already, which have been normal. This definitely rules out any cancers.
You may benefit by the use of antacids as and when needed as well as better acid reducing medications available with prescription from your treating physician. You may also benefit by consultation with your psychiatrist who can evaluate and screen you for any anxiety or stress induced depression, which can sometimes manifest as somatic body pain disorder.
I hope this information here is helpful.