The Hardest Part for Rukia
The hardest part for Rukia is not knowing. It's waiting for something to happen, and it's subsequently dreading that something to happen. The millionth something thronged Rukia's mind so severely that she needed an escape, but no one could give her that. No one could give her escape from this room in the Shrine of Penitence, and no one could still the thoughts her mind has been capable of constructing.
The hardest part for Rukia is a mind numbing knowledge that runs down her bones, that sears her heart. It's knowing something so strongly that it leaves after-effects that don't just vanish as thought swishes to thought, as memory to a different memory, as memories break down and reach a core of a million somethings she has already thought of, and she can't understand it, no, she can't fathom it, this knowledge so deep that she can't even reach it.
The hardest part for Rukia is not knowing, not wanting to know, what she already knows. It's what she fears because she knows he can't reach her; she wants him to forget her and live. It wasn't his fault. It can't've been because he didn't really know either.
She knows this, that he can't save her.
She doesn't believe it, though, because she knows him too well.