此情可待成追忆只是当时已惘然
[cĭ qíng kĕ dài chéng zhuī yì zhĭ shì dāng shí yĭ wăng rán]
The meaning originates from Tang Dynasty poet Li Shangyin's famous work “Love”. It depicts that the moment when one wants to remember past feelings deeply, but it was already gone when it happened. This suggests nostalgia and regret for past lost opportunities and love.