心比柠檬酸情比宣纸薄
[xīn bĭ níng méng suān qíng bĭ xuān zhĭ bó]
The first part means someone's heart feels as sour and uneasy as lemons; in Chinese culture, sour lemons often represent feelings of distress or sorrow. The second part implies that affection seems thin like 'Xuan' paper which is traditionally used in Chinese calligraphy and ink paintings; therefore, this whole saying expresses unrequited love where affection is hard to maintain due to certain situations.