Why is it so hard to get motivated to lose weight?

Im 22 and have type 1 diabetes and coeliac disease. I am overweight and really want/need to lose weight. Im doing this for myself and my health no one else but I just love love love bad food too much. I tell myself that I wont buy it or eat it anymore but I just get a craving and cant stop myself. What can I do? I want to lose around 15-20 kgs and I dont want to do some fad diet where the weight comes straight back on as soon as you stop. Please help....