Welcome to e health forum.
Your doctor is probably treating you for Gastritis or gastric reflux, which is a common cause of chest pain, even in elderly individuals.
Since all your investigations are completely normal, it is likely that you may not have major medical issues. Hence following your doctors suggestions for the time being should help.
In addition, trying to avoid eating high spicy , fat rich foods, which can precipitate your symptoms will help. A healthy diet with lots of vegetables, and fruits should prevent the pain associated with gastritis.
Other issues that cause difficulty with respiration and fatigue can be anemia, and cardiac issues. A echocardiogram should help to confirm that your cardiac functions are within normal.
I hope this helps.