The Myth

"The coherence of the Bible's narrative as a whole is created by a "U-Shaped plot" typical of comedy. That plot begins with the Genesis creation of a harmonious family and garden state, is followed by a fall into a long alternation of historical disasters and triumphs, and concludes with a final ascent back to harmony in the eternal city of Jerusalem at the end of the book of Revelation."
(Northrup Frye's Bible)

We can see the same thing in the Bible or in the works of William Blake:
Creation, the Fall, the Return and the Apocalypse.

This is the myth, in fact the primary myth that pervades all English Literature.
In a large work you may find the myth repeated often.
Look for example at The Book of Job:
The beginning, the fall, the return and finally a future for Job
much like the Return to the City of God.

We find the myth clearly in the story Blake got from the Greek myth of Psyche. .

Blake painted a tempera

that graphically illustrates the basic myth:
Heaven is above; on the right are dry souls, the nymphs;
But they hanker for mortality and descend (Fall); all expect one who chooses to go back, like Thel did! They experience the Sea of Time and Space and a variety of various forms of Fallenness.
But one succeeds and finds the path back to Eternity.

But when once I did descry

The Immortal Man that cannot Die   

Thro evening shades I haste away 
To close the Labours of my Day