FL was mistreated by father and so fell in love with the first guy to show her any form of affection. She used her summoner powers from the shadows to better his reputation. She performs a shady ritual to get more powers for him and that damages her body, eventually leading to her death. Just before dying though, she overhears that guy and the male cousin she trusted talking, and discovers that it was all a setup so they could use her and be together. Time rewinds and she seeks out another man to form a contract marriage with so as to escape from them and get revenge. This other man turns out to be wonderful and they start to genuinely fall in love with each other.
The Duchess with an Empty Soul