Time
She never knows how much time she'll have, on the surface or in the Underworld. At first she knew exactly how long—six months below, six months above. No more, no less.
But somewhere along the line Hades had become restless, doubt filling his heart, and what was once a few missed days turned into much more.
Now Persephone has no idea how long he'll keep her, or how soon he'll pick her up. Her ma had said to take what you could get and make the most of it, but she'd never thought about it applying to time before. This must be what those mortals deal with, she thinks, never knowing when the wind changes on you, or when a harvest might die off.
The goddess of spring doesn't know how they can stand the world's time being out of joint. Persephone knows it's partly her fault, too—she'd accepted Hades coming early and begging her to stay late at first.
By the time days turned into weeks into months, it was too late to fix what was broken.