My case is not as difficult as yours, but I was four when I was diagnosed with asthma. It took me many years to be able to control it (like I can now, fortunately), which meant I took corthicoids everyday for eighteen years or so. I know not all of my weight gain was caused by that, but I would say an important percentage of it was. It also prevented me from growing up as tall as I was supposed to.
I really don't have any useful advice, only to not give up and find doctors who actually listen to you and try to help you basing their treatments on what you can tell them, not some book.