I admire your tenacity! Are you sure it's food poisoning - that's a pretty serious illness which would almost certainly require hospital treatment? Maybe you get traveller's diarhoea very badly. TD is the most common ailment afflicting travellers, and it's inevitable. All water has microbes in it, and your body gets used to the particular mix of microbes in your home supply. When you change location and you get a different mix of microbes and it takes your tummy a while to sort itself out. TD is uncomfortable but not life-threatening (as food poisoning is) unless it goes on for more than a a couple of weeks and you get dehydrated.
How to avoid getting sick: try to ensure the food is fresh. Don't buy anything which may have been sitting in the shop window for an hour or two - ask them to cook you a fresh one. Thai street food is great because it is cooked right in front of you at very high temperatures. Try taking your own eating utensils - often the cutlery, plates and glasses have been "washed" in unclean water. Take hand sanitizer with you everywhere, and use it before eating anything. Work colleagues recently went to Mongolia where the diet is almost exclusively milk and meat - and some of them added more meat and milk to their diets at home in the lead up to the trip, hoping to prepare their bodies rather than dealing with an abrupt change.
Then again, you could just be the unluckiest traveller in the world. Good luck. Phil almost 9 years ago