The fact is, she is now with Yu Zexiao, and she feels very happy. Yu Zexiao is devoted to her, though he has some small flaws in his personality, they are not unchangeable.
As for her and Situ Xuan, there was never really anything between them, and what's missed is missed. She doesn't feel regret, nor does she feel unhappy.
Moreover...
Nie Anni's way of expressing things is also problematic. Who openly criticizes Yu Zexiao in front of his girlfriend, saying he's not as good as other men? As the saying goes, promote harmony, not division. Whether she and Yu Zexiao are doing well is their business, it's unnecessary for others to worry for her.
Relationships are like drinking water—you know yourself best whether it's cold or warm. Maybe Nie Anni thinks Yu Zexiao is not as steady and mature as Situ Xuan, but he's exceptionally good to her, and absolutely devoted.
