Steroid therapy/Rapid weight gain

Due to an severe eye infection (I was losing the sight in my right eye) my doctor prescribed a steroid medication called Prednisone and though it saved my sight, I went from 115 pounds to 185 pounds in 8 weeks. I know how to eat right, I know how to eat to lose weight, I know how to exercise and maintain optimum weight, but now that my eye sight has returned and am off the steroid therapy, the weight will not come off. I am 38 years old and am extremely depressed about this, I went to a doctor and he told gave me Metformim and I am going to try it.
If anyone has had an experience like this, I could use some support. Also, as fast as I gained the weight, I am equally concerned about rapid weight loss as well. THANK YOU