Nature was a biggie for the Romantics, and our man Coleridge was no exception. The speaker in this poem is not talking to us from his living room, after all. He's sat outside, observing the sky and the clouds and the moon. The imagery of these natural surroundings allows him to discuss his dejection—an abstract mood—in a concrete way.
Lines 9-13: The blackness of a new moon is a ready-made symbol of death and destruction for the speaker, one he borrows in fact from "The Ballad of Sir Patrick Spence." In that other poem, it meant that rough storms were on their way to doom poor Sir Patrick while he was at sea. Here, the speaker wishes, essentially, for a similar fate. At least a violent storm would match what's going on inside his troubled head. Bad times.
Lines 28-38: Our speaker checks out the sky, what with all its clouds and stars and, now, a crescent moon. Sure, they all look beautiful, but the speaker can't "feel" the impact of their natural beauty. He's that bummed out.
Lines 48-52: Thanks to the centuries-old language and funky syntax, these lines are a bit tricky to unpack. The gist here, though, is that nature is what you make out of it. If you are in a terrible mood, nature will seem grim and oppressive to you. If you are filled with joy, then all the birdies will seem to sing. It's hard to appreciate nature, though, if you have no love in your life—sniff, sniff.
Lines 67-68: If you have joy in your life, though, that energy will revitalize the way you see, and connect to, the natural world.
Lines 80-81: In these lines, nature provides the speaker with a metaphor in the form of a vine, which represents the hope that once grew around the speaker, but that now is gone.
Line 94: The speaker pulls another symbol from nature. In this case, it's poisonous snakes ("vipers") to represent his disturbed thoughts.
Lines 100-107: The decay and isolation of the natural settings described here really underscore just how bummed out the speaker seems to be. He even throws in a witch's house and a devils' party for good measure, just in case the lonely mountain peaks and dying flowers weren't enough to do the trick.