FL Chooses to Endure Torture / Great Pain and Suffering

To prove that she is worthy of being the Moon Supreme's partner.

The FL tries to maintain a deteriorating timeline all on her own, despite the immense pain it involves, in order to save all the people she loves.  The ML eventually manages to take on some of the pain without her knowledge when he finally starts to clue in.

The FL also transfers poison from the ML's body into her own body, weakening herself and severing her life-and-death bond with the ML. 

The FL endures torture in order to achieve the ML's #1 goal, which will also help her get what she wants. 

The FL chooses to endure extremely painful treatment in order to detoxify poison in her system, because she wants to live. The doctor providing treatment is amazed that she does not scream in agony.