To cease upon the midnight with no pain, While thou art pouring forth thy soul abroad In such an ecstasy! (lines 56-58)
When the speaker enters the nightingale's world, he escapes from the pressures of his regular existence. He's finally happy – happy enough to be content with the thought of dying. The nightingale's extreme joy ("ecstasy") is contagious.
Adieu! the fancy cannot cheat so well As she is famed to do, deceiving elf. (lines 73-74)
When the nightingale flies away, the speaker is left with feelings of bitterness and anger. His happy dream has vanished into the forest. He blames the "deceiving elf" of his own imagination, which has not been not powerful enough to fully create its own world.