What regrets are expressed in Jin Qin's poems?

Li Shangyin's "Jinse" (also known as "Untitled") poem has always been regarded as a mystery in the poetry world. For more than 1000 years, many people have different opinions, which are difficult for poets of all ages. To understand its meaning, we might as well look at the original poem first:

I want to know why my Jinse has fifty strings, and each string has a youthful interval.

Zhuangzi daydreaming, a saint, was bewitched by butterflies, and cuckoo crowed in the imperial spring.

Mermaids shed pearl-like tears on the moon-green sea, and the blue fields breathed their jade to the sun.

A moment that should last forever has come and gone before I know it.

The last sentence "A moment that should last forever has come and gone before I understand it" expresses the poet's regret.