When I have fears that I may cease to be
No, we're not referring to the end of the world or space ships landing back in the 19th century. For Keats, however, contemplating his own end is just as cataclysmic as thinking about the end of the entire world. (Hey, who blames him? Dying probably isn't the best thing to think about all the time.) Contemplating death allows Keats to elevate all of this feelings to a very high pitch, until just about everything is as urgent and melodramatic as, well, the end of the world itself.