Saya feels many things. Regret is not one of them.
This is theme eleven, faith.
Guilt is something she knows well. It constantly creeps up on her with a quiet vengeance, threatening to crush her determination.
It comes when Kyougo embraces her and smiles at her, projecting a passion she knows she cannot, will not return. She feels it when Kotori comes home from school one day, all laughter and childish innocence, proudly displaying a crayon drawing of her perfect, happy family. It's there when Fuuma gives her a look like he /knows/, even when she's sure that it can't be, but then he's always been so aware of emotions other than his own.
In the end, though, it is meaningless because no matter how hard it tries to consume her, Guilt never evolves into Regret. Regret is alien to her, a foreign thing that she will never allow to be. When she looks at Tohru, Regret does not even enter her mind, and Guilt seems far away and insignificant. She sees nothing, feels nothing but /her/. She cannot blame Fate alone for having stolen her heart so, for there is another force at work.
A force that, in a perfect world, would be just as powerful.