Complex sentence: But Kyoko is continually frustrated with what she perceives as Yūsaku's unwillingness to assert himself, as with Kozue And she cannot help but feel jealous, but then feels mortified for being jealous of a high school girl.
Split into simpler sentences: But Kyoko is continually frustrated with what she perceives as Yūsaku's unwillingness to assert himself, as with Kozue. Also, she cannot help but feel jealous, but then feels mortified for being jealous of a high school girl.