Prednisone can indeed, make weight loss extremely difficult. However, since your prednisone ingestion happened so long ago, it is highly unlikely this is the problem.

Removing ALL added sugars and high glycemic foods from your diet, no soda, no juices, no chips or processed snacks, no white bread is a start. Follow a portion controlled, low glycemic, Mediterranean style eating pattern along with at least 30 min of vigorous exercise during the day. The 1200 calories sounds OK, especially if guided by your physician. However, WHAT you are eating may make a huge difference in your success. Sugar is hidden in many foods and you may need to move to only fresh fruits and vegetables, lean meats without prepared sauces, and only minimal whole grain carbs, such as quinoa.

I feel for your frustration, but an increased vigilance for sugar containing foods may assist your progress.