I don't think it's the show/fame she's afraid of losing, there's definitely potential for further shows if she lost weight.

She's afraid of independence. Her entire life all she's known is being dependent on others. The weight is what keeps her family around to do everything for her. If she loses the weight she has to then confront more new challenges. Living alone, providing for herself, maybe getting a job(though with the show and her online notoriety I think that could be avoided), but she won't be able to use her weight as an excuse for her dependence on her family, she'd have to confront the fact that she's not a "bad bitch", she's a 35 year old baby with no life experience or understanding for how the world outside of her bubble works.