?

Sounds like a simple solution and although the author hasn't given a lot of information on this topic, it could most likely be that when she decided to leave her planet it most likely gave her a death sentence, something that can't be cure by just a simple return. What i am trying to say that hours or days outside there specific realm/region etc, the body starts to destroy it self from the inside, and it can no longer be healed. For example, there is a beautiful rare bird that can be found only in Verapaz, Guatemala. It's called Quetzal, it can only be found in humid forest conditions. You cant not take this bird from it's homeland because it will die in days, doesn't matter if you take it for a few hours and than bring it back. The weather conditions outside from it's natural habitat destroys the bird from the inside.It could be fat-fetch by comparing it to a bird, but i don't know their conditions might be slimier. Beings that are unable to leave habitat, bound to one place if they want to live.

That's pretty interesting.

I guess it makes sense that once she left she can't return, I just wish it was clearer.

You will find out in the upcoming chapter(s)! I suspect that even before she left the planet, she was already dying, hence, her enigmatic message about dying with the planet, and that was why she even chose to leave the planet in the first place. If she was going to die, then might as well be happy for as long as she can?