Being a Cornish cross, she's designed to rapidly put on weight, her organs will fail before she's a few months old without a restricted diet, even then a year or so will be about as long as she'll live. I'm sorry.
Chickens are not indoor pets, they need to be outside, with other chickens, for their mental and physical health. Having them inside can also give you serious health issues. (Respiratory and brain issues)
So you really need to to consider another species for a pet.
I don't think chickens would work for you, based on how this one is being cared for, not feeding her chick feed, keeping her in a bucket. That's very likely what's causing her issues.
She *will* eat chick feed if she doesn't have another option, what you're feeding her isn't healthy at all.
She needs, at least a big box as a brooder, not a big bucket she can't breathe in there.
I'm sorry this is happening to you but you really need to rehome this little one and get a different pet, with her breed, she is designed to have a short life.
Good luck find an animal that will fit you best. :]