I'm 55 and OMG the scales this morning said 225.
You stay motivated by realizing that your health should be number one and if you DON'T take care of yourself, things will only get worse and you will likely have less time with your grandchildren.
Talk to your doctor and see if there are alternatives to Prednisone. See if there might be underlying medical causes for the fatigue. Get a complete blood panel done to make sure you aren't battling anything you don't know about.
Aside from that, get yourself to the gym or get outside. I am guessing that if you become fitter (regardless of weight loss), you will find yourself with a lot more energy. Sign up for a few sessions with a personal trainer and learn about how to train your muscles. Heck, if you can afford it, stay with a trainer for a while. Making a large financial commitment is what keeps a lot of people coming back.
Good luck and keep in mind that the only person who can take of you is *you*.
Being fit is a lifestyle change.