Hey, John! Don't get discouraged--you
can accomplish anything you want to. I've had experience with prednisone myself--I was on it for a little over a month when I was 14 and I gained 30 pounds in the space of a month--literally looked and felt like I blew up. But, when a different specialist said I never should have been on it in the first place, I was weened off of it. I was an insomniac, moody and got these awful zit-like bumps on my chest and back...pretty bad side effects for a drug that only masked the problem. I have Ulcerative Colitis, by the way--it's also an immune-deficiency disorder. But Prednisone was bad news for me, so I'm sorry to hear that you're on it...
I can't really talk, as I'm also feeling unmotivated about exercising at the moment--but I can say that it's easier when you have someone to help you. Do you have someone who could be your gym or walking "buddy", to keep you accountable? That might be a good place to start

Once you set your mind to something and believe that you're good and worthy enough to do it, you
can accomplish what you want to achieve.