'And in the dark, I can hear your heartbeat
I tried to find the sound
But then it stopped, and I was in the darkness,
So darkness I became'
This extended metaphor is suggesting ....
because the character can't hear the sound of the heart, s/he feels vulnerable
because the character can't see, s/he feels vulnerable
because the character can't experience the love of the other person, his/her life becomes sad
none of the above - it's something else