When dealing with abstract concepts like despair and depression, it can be tricky to really get your point across to a reader. How does it feel to be depressed? "Bad" or "sad" just isn't quite going to cut it. How about, "like a monstrous lion pinning me down with its terrible paw"? Ahh, that's more like it. Thanks to the wonders of personification, then, Hopkins is able to communicate the depths of his mental anguish in a way that's more recognizable, though also more awful, for us readers.
Line 1: In the poem's first line, we get a veritable turducken of poetic techniques. (That's when you cook a duck inside a chicken…inside a turkey.) The abstract emotion of Despair is personified by being addressed directly by the speaker. This move is also called an apostrophe. The speaker doesn't stop there, though. He then breaks out a metaphor to compare this personified Despair to a "feast" of…"carrion" (ew). The point here is that he's not going to indulge in bad feelings by sitting around and sulking. Even though that might be the most comfortable option, it would be like eating something dead and stinky—no thanks.
Line 5: Once again, the speaker is addressing someone here as "thou." You can make the case that he's talking again to Despair. Or you can make an argument that he's actually addressing God, whom he sees as responsible for dumping all this despair on him in the first place. In either case, the speaker wants to know why this "terrible" addressee is being mean to him ("why wouldst thou rude on me"). Here we see another example of apostrophe—talking to something, or someone, who's not going to give you any answers.
Line 6: The speaker's personification kicks into high gear here. Whomever he's addressing (God or Despair), it's got a super-huge and terrifying right foot, which has the power to shake the Earth ("wring-world") and feels like a boulder ("rock") pressing down on the speaker. Reading a bit further, we see that this is attached to a leg of a giant lion ("lionlimb"). This kind of personification shows just how monstrous and terrible the speaker's depression seems to him.
Line 7: The lion beast's "darksome devouring eyes" represent more figurative language, used to describe the threatening nature of the speaker's despair, as well as his helplessness before it.
Line 8: Personified despair not only has a giant paw and a mean look, but it can also magic up storms ("fan/ O in terms of tempest") to send the speaker's way. This is more than just your run-of-the-mill blues, folks. This is one seriously disruptive (and seriously personified) force in the speaker's life.