Don't say that there is reincarnation of cause and effect, let alone that there are thousands of years of ups and downs in the world.
(If there are a thousand Hamlets in the hearts of a thousand people, how many sycamores will there be in the hearts of a thousand people? The story is told in the first person, there is no emotional line, there is unrequited love, the timeline is uncertain, write whatever comes to mind...)
Concept: Apocalyptic science fiction