Two words:
Miss, recall, recall, meditate, meditate, recall, unforgettable.
Four words:
Sadness, bitter thoughts, memories of the past, myriad thoughts, memories of the past, flowers in the evening, fresh memories, unforgettable memories, unbearable memories, a feeling of the past and the present, a feeling of learning new things by reviewing the past, endless aftertaste and unforgettable memories.