Hello, I'm so sorry to hear about Dizzy and I'd like to help.
Unfortunately dogs with Cushing's disease (hyperadrenocorticism) are very prone to secondary infections because of the increased levels of cortisol suppressing their immune system. I suspect that may be the trouble with her. Skin infections can make them very, very itchy. While some normal dogs with skin yeast infections can have their disease controlled with just shampoos and topical treatments Cushingoid dogs may need oral anti-fungal medications as well. I would ask your veterinarian about using an oral anti-fungal medication as well.
I would also be concerned about parasites because of her immune system being suppressed, such as Demodex or Sarcoptic mites. Both of these mites will make her quite itchy and will thrive in a dog with a suppressed immune system. I highly recommend skin scrapings or maybe even a skin biopsy to make sure that these parasites aren't part of the reason that she is so itchy and miserable.
I would also add an omega 3 fatty acid supplement, unless she has fish allergies. The dose is based upon the EPA (eicosapentanoic acid) portion of the supplement because if we dose that way the others will be properly balanced. Give 10mg to 20mg per pound or 20mg to 40mg per kilogram of body weight of EPA per day. Most supplements will list the ingredients (and if they don't it likely isn't a quality supplement) and EPA amounts should be listed. For example if the supplement has 300mg of EPA per dose then that dose would be perfect for a 15 to 30 pound (7 to 14 kilogram) dog.
I don't know how frequently that you are bathing her. Some dogs need to be bathed daily or every other day or at least spot treat the worst areas until the infection is under control and then decrease it to the least often bathing that will control the situation. I would make sure to follow up with a conditioner as well to add moisture back to her coat and skin.
Best of luck with your girl, please let me know if you have any further questions.