Maybe this is not the most helpful answer but have you figured out what foods cause problems for you?
Related to that, these diets are good for treating food intolerances (and/or allergies):
- Gluten free - see How to eat gluten-free for chronic illness
- Low histamine diet - see Low histamine diet - what it’s good for + food lists
- Carnivore diet - see Meat only diet - Long Haul Wiki
There are probably other diets that work but there isn’t a lot of information on them.
And then regarding exercise… you could look into pacing strategies or changing how you exercise. Welcome to the forum by the way.