念念不忘耿耿于怀
[niàn niàn bù wàng gĕng gĕng yú huái]
It conveys the idea of something persistently on one's mind, never forgetting or moving past it easily. '念念不忘' indicates constantly thinking of something while '耿耿于怀' adds emphasis on not letting go or putting something out of mind, both reflecting deeply cherished or possibly unresolved issues.