What's it about?
This poem is about the end of a romantic relationship. It was enjoyable while it lasted ("We took our hearts' full pleasure"), but now it's over. The couple in the poem are like boats ("ships", "argosy") at the end of a voyage - they have rolled up their sails ("sails … are furled") and stopped taking more cargo on board ("spent the lading").
In the middle stanza, the poet describes his sadness ("sorrow"). His cheeks and lips are white ("wan", "paled") and all his happiness has gone ("gladness fled"). He imagines that ruin has come to him and closes ("draws") the curtains around his bed, isolating himself from the world and the people around him.
In the third stanza, the poet tells us that his former lover isn't as badly affected as he is. He accuses his lover of not taking the relationship seriously.
He says it was no more real to them than the sound of musical instruments ("lyre", "lute", "viols") that feels magical ("subtle spell") for a short time. It wasn't a real relationship, but just an "echo" of one, like the sound of the sea in a shell.