She accepts it due to the inescapable reality they find themselves in. One of the cornerstones of her outlook is a belief that one must look honestly and truthfully at the world one lives in. When she speaks of her own revelation she describes how “freed from presumption or prejudice, I saw the world through a newborns eyes. Everything fresh and new and so, so beautiful.” It’s why she doesn’t reject our story when Emet and Hythlo did and it plays into how she responds to Meteions breakdown, with her stating that Hermes has a right to hear Meteions report but that they should be there as well so he isn’t alone.
So rejecting the premise to her is the equivalent of rejecting gravity or time or any other constant. And that’s very different from believing her world needs to suffer out of preference for that state. One is a begrudging acceptance of the nature of existence, the other is a belief that it is right and good that people should suffer. Perhaps its not an important distinction in other peoples view, but it’s one that I think helps to define her character. The Sundering was a necessity in order to overcome the challenges ahead, not a good in it of itself.
I think that quote shows her struggling with a specific cultural norm, rather than being indicative of her distaste for the state of the world then. In her speech to us she actually reaffirms the Ancients way of thinking, stating that as soon as she feels she’s accomplished her purpose she’ll return to the star. It’s just that that purpose has a much longer path than the others. If she had felt that mankind can find it’s own way before stepping down, I don’t think she’d even be voicing that frustration.
I’m a particular fan of that speech, as well as the one from the (in)famous cutscene after Elpis. The way she describes how she came to love humanity is I think, regardless of anyones feelings on her beyond that scene, beautiful.