The last two lines are the "turn," the moment where the focus shifts -- in this case, to that oh-so-Romantic notion of sublimity. The narrator's been worrying about dying because he might not have a chance to write everything he wants to write or experience everything he wants to experience or love for as long as he wants to love ... but that fear makes him realize the vastness of the world, which is awe-inspiring enough to make all his worries irrelevant.