Three Words or Less
Shizuru can't shake the feeling that this could all be resolved if she could just find the right words. "I'm sorry," maybe, but she's tried that already, and all she got was a shake of the head and a smile that made her want to cry. Maybe, "Goodbye," but she's not selfless enough to face missing Natsuki, not selfish enough to leave just so that Natsuki misses her, just so that Natsuki thinks of her.
There's always the chance that, "I will stay," and deciding to go to Fuuka University would give them the time to figure themselves out. But Shizuru knows she already has herself figured out, will not and cannot change; knows also that she will put up her best front to avoid confusing Natsuki, and so will confuse her even more. The extra time would probably make no difference, at any rate. Natsuki's words during the Carnival sounded too much like "cannot" to be mistaken for "do not." At least they weren't "will not." Shizuru likes counting small blessings, because there are many more of them than large ones.
She really should be bracing herself to say, "Let's be friends," of course, but that sentiment is already a given, and she can't bring herself to add the "just." Can't even decide where in the sentence it should go, because that would mean thinking about it, which would mean giving up. Shizuru, for all her grace, is bad at giving up, bad at giving in, cannot lose hope because it is one of the last good things she has. It is also the most painful.
Perhaps "I cannot stay" would draw some sentiment out of Natsuki to fill the silence she's sure would follow. Perhaps that sentiment would give her some idea of what to do next. Unfortunately, she knows that Natsuki has no more idea of how to fix her than she does herself. (Her own theory is that she will be fine if she forgives herself, but that seems- beyond impossible. What kind of selfishness does it take to forgive selfish weakness merely to heal yourself? And yet she hurts others - Natsuki - by holding her guilt tightly, too.)
"Maybe one day" is impossible, just as impossible as "Let's be friends," but in reverse. She refuses to think of it, too.
What is left?
"Natsuki," she says, light accent drawing the name out into an endearment.
The blue head turns, seemingly permanent scowl dissolving into something that is almost a smile as an eyebrow lifts in her direction.
She forgets what she decided to say.
Instead she merely smiles back, and sips her tea as if she has all the answers, because she is Shizuru and she does. It's just that she has all the questions, too.