Thus, the poem is the history of the failure of a man with an artistic temperament to be an artist.

Or rather, that is part of the story of the poem, and, as Browning was an artist himself, a part which is of the greatest interest.
Sordello, at the close of the first book, is wearied of dreams.

Even in his solitude, the limits of life begin to oppress him.

Time fleets, fate is tardy, life will be over before he lives.

Then an accident helps him-- Which breaking on Sordello's mixed content Opened, like any flash that cures the blind, The veritable business of mankind.
This accident is the theme of the second book.