This poem plays a lot with night and day, light and dark. Pretty much anything that has to do with the present is going to be dark, depressing, and related to the night. But when the speaker thinks back to getting hot and heavy with his lover, then the fire starts blazing! Watch the way the imagery illuminates (get it?) the poem's message.
Line 1: The poem takes place in the night, and this line makes us focus in on the memory of the speaker, as though it were a point of light in the dark. That memory "emerges" here, like a person might, is a case of the speaker using personification.
Line 3: The moment right between darkness and light (dawn) gets a lot of attention in the poem, and it is characterized as a lonely time. This is the time that the speaker, in a simile, seems himself as deserted as the wharves.
Line 12: The memory really starts to burn here. What was a little memory emerging from the night is now as bright as a beacon. We get another use of simile here, too.
Lines 19-20: The poetic speaker tries to get on with his life, as tough as it might be for him. He describes that in terms of darkness and light here. The heartache that he tries to move past is a "wall of shadow" that he pushes away. Both in terms of actions and desire, he tries to move on—presumably into the light.
Lines 25-26: Islands are lonely things (not even an isthmus to hold on to!), even metaphorical ones. When they're dark, the fact that they're plopped in the middle of the big ocean is an even lonelier thought. The woman saved the speaker from that dark loneliness.
Lines 34-35: Even though the love has ended, the speaker still relates the woman and love to light. But instead of being a guiding light, like a lighthouse, here it is a consuming fire.
Line 56: The shadow, which (like any shadow) is cast by blocking the light, is the only thing left for the speaker. The woman, who has been identified by light throughout the poem, is separated from the speaker by time and space, so this metaphor shows us that he has only her shadow to remember.