The river mingles its stubborn lament with the sea. (2)
The river is a metaphor for the speaker, who is using the poem to lament his loss, just like the river's sounds are like a sad song (lament).
You girdled sorrow, you clung to desire, sadness stunned you, in you everything sank! (17-18)
The woman was expected to save the speaker from his sadness, but it seems as though she was trapped in it, too. In fact, she seems like she was paralyzed by it.
There were grief and the ruins, and you were the miracle. (28)
The woman was to be the miracle that magically erased the sadness for the speaker, but maybe since they "found love in a hopeless place" they were doomed.
Oh the mad coupling of hope and force in which we merged and despaired. (37-38)
The love that the couple felt for each other was a crazy one. It seems that they tried to come together in their sorrow, but that the sadness was stronger than they.
what sorrow did you not express, in what sorrow are you not drowned! (44)
Hmm. Deflect much there, speaker? Does the speaker blame his lover for being too weak to overcome the sadness?