nothing sadder than a withered heart (idiom attributed to Confucius by Zhuangzi 莊子|庄子[Zhuang1 zi3])
no greater sorrow than a heart that never rejoices
the worst sorrow is not as bad as an uncaring heart
nothing is more wretched than apathy
Definitions (粵典–words.hk)
語句
最悲哀嘅事莫過於死心,再唔會有希望或者關心啲乜 nothing is more wretched than apathy
Definitions (Wiktionary)
proverb
Despair is the greatest sorrow; Nothing gives greater cause for sorrow than despair; There is no grief so great as despair; There is no poverty like the poverty of spirit.