蓦然回首你已不在身后
[mò rán huí shŏu nĭ yĭ bù zài shēn hòu]
It reflects a moment of surprise and regret: when turning around unexpectedly (or after a while of absence), one finds that the loved or missed person has already gone away. This phrase originates from classical literature and conveys the feeling of losing something important unintentionally.