此情不堪回首
[cĭ qíng bù kān huí shŏu]
It means 'This Love Can't Bear to be Recalled'. It is often used when describing past loves or relationships full of sorrow. It implies a deep, regretful feeling where thinking about those days will inevitably trigger pain or sadness. This can also reflect the owner's current sad mood or situation about an unrequited love affair.