Certain medical disorders or medications can cause weight gain/inability to lose.. You might talk to your doctor to see if there are alternative meds you can take that might control you disorders, without causing weight gain.
In my case, I have hypothyroidism, which slows my metabolism -- if my thyroid levels aren't right, there's no way I can lose weight, because thyroid controls metabolism.....
it's not an easy thing to deal with because it takes getting meds balanced so they control one thing, without affecting something else......
Well you need to monitor your food intake which means to chnage it up to a health diet. There i a certain calorie intake when on a diet but that depends on your height and age also. Try cutting out carbs and eat more protein, fruits, and veggies. Exercise if you can will speed up your weight loss.