One single slip brings eternal regret, and looking back, your whole life has passed away.

一失足成千古恨，再回头已过百年身。

《新英汉大辞典》

Fault is the occasional mistake, but miss is eternal regret.

过错是偶尔的失误，但错过却是永远的遗憾。

youdao

This is an eternal regret handed down from generation and you are only one of those who languish for flowers.

这是我们代代相袭的永恒的遗憾，而你只不过又是一个为这朵玫瑰而憔悴的人。

youdao

To my eternal regret.

NEWYORKER: A Dirty Business

Some 57 years later he wistfully remembered the regret at leaving the eternal snows of the Jungfrau and the sharp outline of the pyramid-shaped Silberhorn peak against the dark blue of the sky.

BBC: In Alpine villages, Hobbits lurk