When I have fears that I may cease to be
The regular meter of this poem, iambic pentameter, means that it tends to flow pretty smoothly over the tongue. (Read about that in "Form and Meter.") Ironically, that strict metrical pattern sounds a whole lot like regular spoken English. In other words, Keats is playing with complicated systems that end up sounding a lot like spur-of-the-moment thoughts.
In fact, much of Keats' language is far more carefully constructed than it initially appears. Elaborate natural metaphors are underscored by a whole lot of alliteration. Just take a look at the first quatrain: "glean'd," "grav'd," "garnered," and "grain" all feature hard "g"s, tying the speaker's metaphor together with one constant sound. Just like the carefully constructed iambic pentameter, Keats' alliteration stacks up without our really noticing it. If you're not paying attention, it would seem like the speaker just happens to think in really elaborate, haunting phrases.