Cushings is the exact opposite of Addison Disease, unlike Addison whereas your body lost the ability of producing proper cortisol, Cushings produces much more than your body needs.
The cause of my Addison Disease stems from ME/CFS and not by using any steroids at all. The following site hopefully will help you undersand Cushings a bit more.
Cushing's syndrome is when you have too much cortisol. It can be from a tumor or from taking steroids as prescribed from a doctor. If you take steroids for too long, your adrenals can atrophy and then stop working, then leading to adrenal insuffienciency. I have AI due to having my adrenals removed to cure Cushing's disease.
Now you must take steroids to live for the rest of your life - and you need to regulate your salt, learn to stress dose, etc. Too much as well as too little cortisol is bad. Too much is damaging to the body and too little can lead to an adrenal crisis. Look up information on Addison's to know what to do.