蓦然回首怎奈梦已空
[mò rán huí shŏu zĕn nài mèng yĭ kōng]
This phrase comes from an ancient Chinese poem. It can be loosely translated as 'Suddenly looking back, but only to find the dream has already turned into emptiness', implying that it is regretful when trying to recall past events after a long time.