one who has seen the world or reached a high level of expertise is no longer impressed by trivialities or mediocre things

他已经见识过顶级的艺术作品，如今回头看这些小品，不免有曾经沧海难为水，除却巫山不是云的感觉。

once one has experienced the greatest or most beautiful things, nothing else can compare or measure up (often used to describe devotion to a lost love)

自从他失去了初恋，便觉得曾经沧海难为水，除却巫山不是云，再也没有人能走进他的心。