a matter that gnaws at one's mind; preoccupation; sore point; rancor
Definitions (粵典–words.hk)
名詞
令某人受困擾嘅事（量詞：個） a matter that causes distress/unease in a person (classifier: a)
佢哋一直都放唔低呢個心結。
佢哋一直都放唔低呢个心结。
keoi5 dei6 jat1 zik6 dou1 fong3 mm4 dai1 lei1 go3 sam1 git3 All along they have been unable to let go of this knot in their hearts.
Definitions (Wiktionary)
noun
emotional matter that gnaws at one's mind; issue that weighs heavily on one's mind; complex; rancour; preoccupation (syn.) 苦衷, 隱衷, 心曲, 隐衷, 心事, 衷曲