Is an easy way to add fiber, but it’s probably best (an much less expensive) to get fiber from food like beans, lentils, farro, grains, fruit & veg (especially in place of meat and dairy) Basically, if you can avoid most animal products and processed foods, you’re going to be fine! You’ll be getting fiber all the time.
Sleep and physical activity are key, too.
I am not a doctor, dietitian, nutritionist or in anyway qualified to give advice. This is just stuff I do.
Also, stomach issues can be from anxiety and I don’t know if doctors take that seriously yet. What helped my stomach more than anything was Wellbutrin.