I would recommend to work her and get her tired ( hikes ,play, fast spurts)etc.
Set a schedule feed only small amounts and buy the smallest bag.
Good advice in general, but definitely get the food right and make sure there aren't other underlying issues.
As a side note, I had my first dog on a prescription food for awhile. This was long beforeallergy" foods were available in the stores and before there was such a great variety offered. It was a Royal Canin food, and I had some bags that were "off" and weren't manufactured properly. I never realized it until I got the new bag and my dog got better. So, remember, ALL processed food is subject to poor processing which may affect the dog (or cat).