To add further to that:
1. Visit the Equine Cushings & Insulin Resistance Group
There's a lot of good information and help on that website. It is free to join.
2. Absolutely no grain and the mare may also benefit from no-soy (soy is used in 99% of feeds & ration balancers as a protein source.
3. Everything as low starch as possible because, when you add up the cumulative NSC of hay, grass, feed, it can be well over the acceptable limit for a horse with PPID and/or metabolic issues.
4. You may have noticed the mare tires faster. It has been my observation with my metabolic horses and my friends' horses with both Cushings/IR that they start out with normal energy and all of a sudden it's like someone punctured the balloon and they quickly lose energy.
Mild exercise is great as long as she is ridable.
5. Might also need to shorten the time frame between hoof trims -- even if it means the farrier has nothing to do but rasp and keep the hooves from flaring.
Mine are trimmed every four weeks. Depending on growth rate your mare might be ok with 5-6 weeks.
6. No sugar treats. That includes carrots and apples. There's more sugar in carrots than apples.
6.1 Buy quality grass hay - locally grown horse hay if that's possible. First or second cut doesn't matter at all -- it's time of day
the hay is cut that matters for keeping the WSC & NSC low.
7. How many years a horse can go with cushings depends on each horse and also the care it receives. Some live well into their late 20's/early 30's, while others might only make it a few years.
Early discovery and treatment also play a role. Some people live in denial for a long time before getting help and thus shorten the horse's life.
Read credible articles, ask questions. Knowledge & treatment methods have come a long way since the horse in my avatar was diagnosed with Equine Metabolic Syndrome in 2007. EMS is more of a relative of cushings than insulin resistance.
He lived seven more years but it was strangulating lipomas that took him, not the EMS. I had the EMS well under control and he had never foundered, never needed a grazing muzzle but I did shorten his pasture time.
Keep asking questions, what works for one horse may not work for another or the dosages may be different. You may have to experiment at first, to get diet and drugs regulated but just take a deep breath, throw your shoulders back, one foot in front of the other, and before you know it, things will level out.
However -- you will always have to keep one eye on alert for even the slightest change in the horse:)