Complex sentence: Her relationship with Chuck changes as he tries to win her back and she tries to own up to her own feelings towards him, Blair becomes tired of the high school pettiness until the her admission to Yale is compromised.
Split into simpler sentences: Her relationship with Chuck changes as he tries to win her back and she tries to own up to her own feelings towards him. Blair gets tired of the high school pettiness until her admission to Yale is compromised.