I know what you’re going through. My Jax is like your Bella: he doesn’t eat unless he’s hungry, so I have to leave his kibble down for him to free-feed. I also have an old dog who, while she has never been super food motivated, does consider Jax’s kibble to be a highly desirable treat, so I have to keep her away from it. So what I do is keep Jax’s kibble in an x-pen and put him in there regularly to eat if he’s hungry. It’s not a perfect solution, but it works pretty well.
I have a friend who has several cats on different prescription diets, and she uses these:
https://www.amazon.com/dp/B00O0UIPTY...ogi&th=1&psc=1
They’re pricey, but she says they work very well. I think they would work for a small dog, too.
But don’t think you’re a bad mommy if you have to keep doing what you’re doing now. Sometimes you just have to make the best choice you can and go with it,
One minor thing: I would ask your vet about the parmesan cheese. It’s awfully salty to use regularly.