I have owned many cats in my life, so I will put in my 2 cents worth. I agree that taking her to the vet is a good idea. Extra attention might help. But I had a cat that was outdoors only growing up. As she got older, we started letting her in on cold nights, and when she ate. She didn't like the litter box either. Why? Because she was use to dirt. Started filling the litter box with good old dirt from outside, and she used it. You have to change it every day because of the smell, but maybe worth a shot?

I really hope you can get this problem solved. I know how frustrating it is.