There is no next sentence. The original sentence is: Old dreams can be revisited, but broken mirrors cannot be reunited.
The meaning of the whole sentence is to describe that although in reality, couples are forced to separate due to objective reasons and cannot look back, but in dreams, they can experience the past scenes again.
Broken mirror and reunion is a Chinese idiom, which comes from Tang Dynasty Meng Qi's "Poetry of Poem and Emotion". This idiom refers to the reunion and reconciliation of a couple after separation or breakup. Its structure is in the form of a main predicate, which can be used as a predicate or object in a sentence; it has a commendatory meaning.
Reunion is not limited to the allusion itself. Breaking refers to the formal or internal separation caused by subjective or objective reasons; reunion refers to the further development of feelings through the subjective efforts of both parties, and finally reaches formal or internal perfection or completeness.
Emotions develop and change, so breaking is just a state, not the final state of cessation. As long as the feelings for each other are still there, through subjective efforts, the feelings can be further developed and eventually made up.