Resource Guardimg

Resource guarding...how do we make it stop. My female corgi is very intelligent but she guards people, locations, items, etc from our male corgi. It's so random what she will guard. Both are spayes/neutered. We got her as a puppy first then got him as a puppy 1 year later. They got along great until he became bigger than her. We have reprimand her every time gotten tried Prozac and that didn't help. Switched her diet to grain free, corn starch free and chicken free. Nothing is helping and it's getting worse. She is only 4 years old. I don't want to give up either of the dogs but we are constantly yelling at her to stop growling.