My horse is extraordinarily fat..... Diet plans?

It's time to take her of the Strategy, with a horse that fat you don't need it. You also should get a grazing muzzle where she can't eat as much day. You can also ask the barn owner if you can use the arena to put her in while no one is using it so she can't just much down the hay. It's also time to work her harder then you're working her. Make her sweat every ride, give her real work.