When Nathaniel is with Clara and Lothaire he read the poem he wrote to Clara. What was the poem about that was so upsetting for Clara?

Nathaniel had once been a great storywriter and bonded with Clara in his youth by telling her these stories, but he now begins writing and reciting for her long pieces full of "dismal, obscure, tedious mysticism." He writes one such piece, a poem in which Coppelius destroys his and Clara's love, reaching in and tearing out of their lives anything pleasurable, causing Clara's eyes to fall out and burn Nathanael.

After reciting this piece for Clara, she tells him seriously to throw "the crazy senseless, insane story" away. He pushes her away violently and yells at her. Lothar finds out about this occurrence and the two men decide they must duel. However, Clara interrupts their duel, telling them to kill her before killing one another.