Part of it is that — to be perfectly honest — she's also a child. We've seen repeatedly that she's not much more mature than Mashiro.
But really, she had this sprung on her, and she's very confused about what she should be doing. She really should, as everyone has said, focus on just being friends with Arata, but the Hakuya/Kobeni relationship still has her feeling competitive, as does all this cultural baggage about what a "couple" should be like. She really doesn't know what "progress" would even be at this point, and the age gap makes that worse.
(That and I'm not entirely convinced she's heteroromantic, rather than wanting "a future husband" in cultural programming ways distinct from personal desires.)