Forlorn! the very word is like a bell To toll me back from thee to my sole self! (lines 71-72)
The middle of the poem takes place in the forest of the nightingale, but the speaker's imaginative spell is broken by the word "forlorn," which announces that the bird has flown away. In other words, he got rejected. Now he's stuck again in his own mind. Oh, great.
Was it a vision, or a waking dream? Fled is that music:—do I wake or sleep? (lines 79-80)
The speaker gets that feeling you have just after you wake up from a very persuasive dream, where you're like, "Wait, so am I really the President of the United States, or not?" He cannot distinguish fantasy from reality, and he remains in a state of confusion.