1. NO GRAIN
2. Put on a high quality bagged feed for these types horses or a ration balancer. I would prefer a ration balancer that is vit/min fortified.
3. Grazing muzzle for turnout or put in a dry lot with weight-measured hay (that would be dry before you soak it).
3.1 If dry-lotted, scatter the hay around so she has to move because she needs exercise.
This Southern States link talks about feeding insulin resistant horses, but it's the same game plan for a cushings horse.
Feeding the Insulin Resistant Horse - Southern States
I must respectfully disagree, cushings horses do not follow the same game plan as insulin resistant horses. My horse has been diagnosed with and treated for cushings for the last 7 years. He gets grain every day, in fact he is given cob and a mix of equine senior. He is also fed ten lbs of alfalfa hay twice a day. This would probably do great harm to a truly metabolic horse, however cushings is in a sense a whole different animal. The best plan for you would probably to have some tests done perhaps by a vet who is well read in the syndrome. They can give you a much better feel for the correct feeding plan.
It is of upmost importance that your horse is diagnosed with the correct disorder, cushings is no joke and your horse should be treated right away, my horse has been on Pergolide for 7 years, he is now 28 and has never looked better, no joke people can't even fathom he is as old as he is. I have also seen metabolic horses be prescribed this drug, and it was very effective.
People can get hyperpituitary too & it is treated with surgery through the nose. Sadly, there is no easy access to a horse's pituitary gland or they could be cured too.