John Keats reveals his fear that he will die young, and before he gets the chance to experience the love of a wonderful woman and also before he reaches the pinnacle of the talent as a poet. Oddly enough, he does die early--at the age of 25--from tuberculosis. He is buried in Protestant Cemetary in Rome.
This sonnet is divided into three quatrains and a couplet. In the first quatrain, he focuses on his fear that he will die early before he has the ability to write about all the things running through his creative and inspired mind.
In the second quatrain, he worries that his imagination and inspiration will run dry. This is a death of sorts, too, especially to a poet and writer.
In the third quatrain, he ponders the possibility of never finding his soul mate or having a romantic love affair. He desires a love that would fulfill him as perhaps his writing and inspiration from nature has thus far in his life, and fears that this will never be.
In the final couplet, he recognizes that what is meant to be will be. We can never know what is in store for us, but the plan of a universal higher power is inevitable. We all have hopes and dreams and we all face mortality. His fears that his life may cease and with it the moments of inspiration which propelled him as both poet and lover leave him deflated. He is alone in all the "wide world", lost in thought, sinking into "nothingness".