If she's extremely fat, I'd consider a basic diet of grass hay, pref low NSC tested or soaked to leach out excess sugars, &/or restricted pasture, using a grazing muzzle or such. I'd avoid 'complete' feeds generally, as well as grain, molasses & anything high energy/carb.
There are some good grain free, low carb, low dose 'ration balancers' available, such as KER Gold Pellet, Kelato Performance, Kohnke's, etc. If feeding only soaked hay, powdered supps can be sprinkled onto wet hay.
FeedXL.com is one good resource for working out what/how much your horse needs. Studies have indicated that extra magnesium may be beneficial for IR/obese horses. Check out safergrass.org for some good general feeding info, esp re fat horses.
Re bran/digestive issues, I wouldn't be feeding wheat bran regularly, unless you're using it to balance deficiency in phosphorus. Rice bran is high energy, so probably not suitable either. Consider treating for ulcers & giving probiotics to help her GI tract recover from probs that may have arisen from such as too little/infrequent feeding of roughage, starchy, sugary feeds like grain, bute, antibiotics or other medications. You may also want to test her for insulin resistance(sim. to type 2 diabetes) & if she is, cinnamon has been scientifically shown to increase insulin sensitivity.