Guilt is one of the symptoms and stages of grief. It is probably one of the toughest stages to deal with. It sounds to me, however, like you have no reason to feel guilty. A healthy animal will never starve himself. Even if he does not like the food, he will eat it after a few days of being offered. Your cat was ill, as your vet told you, and you did everything you could to try to help him through it. Allow yourself time to mourn and remember, guilt is a normal stage of grieving. I remember how much that stage hurts. I've had to go through it three times just in the past two years. It's painful. But it was not your fault. And once the guilt passes, you'll probably see that. I'm sorry for your loss.