It's not that she couldn't bring herself to say she hated Sehmin's confession, its that she intentionally said she liked it despite hating it as a way to push Doojoon to do similarly so that he can get rejected and stop being so lovesick for Hana. Sehmin's really the one who was the worst to get advice from because his approach is "be so forward with someone who doesn't know you so that they instantly become very uncomfortable with you and then keep trying to pressure them".