The speaker is addressing a dream, or perhaps a person who has turned into one, that lives only inside their head. At first they plead with it to listen and to leave the "closet" of the mind, as if longing could coax it into the real world. But the rest of the poem suggests it never fully arrives.
The images get lonelier as they go. The dream is an island that receives the speaker's waves, yet has "no place to land," so the feeling keeps reaching toward something it can't settle on. Then it becomes a wind that brings a "sweet breeze" and never returns, leaving the speaker cold. The sweetness is real, but so is the loss.
The mood is wistful and quietly aching. There's no anger, just a sense of reaching for something beautiful and out of reach, and being left with the cold afterward. The title, "Fallen angel," hints at that same mix of something lovely that has slipped away.