One part of her anxiety is more learned helplessness than actual laziness (Though of course she is also lazy. It's hard lugging 600 lbs around.). She can not do things on her own. She never had to learn. So of course the prospect of being alone gives her huge anxiety. I also think that she has problems with stabilizing her emotions when alone. She is an emotional void with no true sense of self that lacks the ability to self-sooth. People like that can never be alone.
The other part is actual health anxiety. All deathfats know they are just one misstep away from serious health issues or even death. Even if they deny it (thus the meltdowns when they have to deal with doctors), they know it. She is afraid she will die in her sleep or fall or have some other health emergency and then who will take her to the ER?
Also good Lord, the only thing about her body that is still vaguely human is her giant moon face.