@Countrygirl18, others have made some excellent suggestions about things to try to solve the diet problem, so I can’t add anything to that. But, I have a couple of questions.
First, what age is your cat? If she is middle aged or older, have you ruled out hyperthyroidism? Hyperthyroidism can cause itchy skin, which can lead to excessive licking and hence to hair loss.
Also, have you ruled out flea allergy? Flea allergy is less likely if she is indoor only and an only pet, but if she goes out or is exposed to other animals this could be the problem. And it could be fleas even if she’s on flea medication, because fleas still get on her before they are killed. Also, the fact that the hair loss started in March suggests to me that this could be fleas or some other seasonal allergen. If it clears up this winter that will pretty much make the case that this is seasonal.
I once had a cat with hair loss like you describe. With my cat, it always started in the spring and cleared up over the winter. And it may be coincidental, but she was eventually diagnosed with hyperthyroidism. It may just be coincidental, but after she was cured of that with radioactive iodine therapy she never had any hair loss again.
IIWM, as long as the cat is otherwise healthy and happy I wouldn’t fret too much about the hair loss. A diet change is worth trying, but I wouldn’t make her miserable about it if she’s determined not to eat it.