How does the poet tossed back from ecstasy into despair?

While Keats praises the bird's song, the nightingale flies further away from him. The poet sighs, “Forlorn!” As the sound is distanced, the spell breaks and Keats awakens to the reality. He realises that while dreaming he had travelled far from reality. He is thrown back to the physical world again and he grieves that his imagination is not to become reality. Keats laments at the loss and wonders whether he was dreaming or he envisioned it all. He bemoans for he is bereft of his art and the beauty.