I'm not sure whether you have high stomach acid or low stomach acid, but I do believe that you have high acidity throughout your body. Once you clear out the acidity and re-balance your body, your stomach acid levels will also be in balance. You need a diet high in simple carbohydrates and low in oils, fats, salty, spicey foods. When you eat large amount of simple carbohydrates you may have some symptoms of burning as acidity is being removed from your body, but you should notice a decrease in heart burn. By the way, when I say "a large amount of carbohydrates" I don't mean you should overeat, you should eat only the amount that you have an appetite for. After you eat this way for some time, you will notice that your appetite will increase greatly, then you can start eating more and putting on some weight. Never try to force yourself to gain weight, this throws things more out of balance. When you eat the right foods for your body appetite and everything will fix itself.