Hi, Alison,
Cushing's Syndrome, or Cushing's Disease, responds only moderately well to treament. When complicated by other factors such as you describe you really should consider your pet's quality of life, and ask for whose benefit are you prolonging that life - yours or hers? If hers, is she REALLY getting any benefit?
The price of enjoying the love and companionship of pets is that their life expectancy is so much shorter that ours, and we must go through the grief and trauma of losing them several times in our own life. I think that it is important not to let the grief at parting outweigh the pleasure and love given to us by our pets.
If you are in any doubt, ask your vet to advise you. To have a pet put to sleep is a hard choice to make, but a good veterinary surgon will be able to help you reach the correct decision. The Bluecross animal welfare charity also has a Pet Bereavement Service which offers counselling at this time.