I love Doctor Who's writing. Donna Noble in particular made the show notable.
Spoilers About Doctor Who Ahead:
One of the things that made Donna Noble losing her memory of the Doctor so tragic was that she forgot about who she was when she was with the Doctor. Throughout season 4, Donna grew as a person who learned to see how amazing she was and move passed her mother's harsh words. She lost that version of herself when she forgot about the Doctor.
I think her fate was worse than any of the Doctor's companions because she lost the most. She lost adventures. She lost the Doctor. She lost herself.
That is what should happen at the end of your novel.
It should be tragic for your hero to forget their adventure and lose their character growth.
Your character must grow. At the end of your novel, your character should not be able to return to who they were at the beginning.