无法预知的伤痛难以回首的曾经
[wú fă yù zhī de shāng tòng nán yĭ huí shŏu de céng jīng]
Translated to 'Unforeseen pain, memories too hard to revisit'. This implies past traumas and painful experiences that one finds difficult to revisit mentally or emotionally. A deeply reflective and melancholic sentiment.