Before my pen has glean'd my teeming brain, (2)
Our speaker's ambition comes in a strange form: he's not worried about whether or not he'll write good stuff. In fact, he's sure that his poems will be hits. He's just worried that he'll die before he has a chance to get them all out on paper. It's almost like writing isn't a task at all – his pen can just pull words from his "teeming brain!"
Before high-piled books, in charactery,
Hold like rich garners the full ripen'd grain; (3-4)
Yet again, Keats' speaker is given over to the possibility of unrealized ambition. He wants to write lots and lots of books full of poems like newly-harvested wheat. (Yes, you read that right. Poems = wheat. Get it?)
--then on the shore
Of the wide world I stand alone, and think
Till love and fame to nothingness do sink. (12-14)
It's almost like a manic version of Buddhist philosophy. Worried about all the trivial stuff in life? Go off by yourself for a while and put things in perspective. Why do we call this manic? Well, we get the sense that Keats likes both the rush of desire and the rush of annihilation. The tension between the two is what we like to call "negative capability."