Complex sentence: She is in love with her childhood friend Yū Setoguchi but is too afraid if he rejects her, their friendship will be ruined, thus lying to him that she's using Yū to be her confession practice partner.
Split into simpler sentences: She is in love with her childhood friend Yū Setoguchi but is too afraid if he rejects her, their friendship will be ruined. So she lied to him that she's using Yū to be her confession practice partner.