Death. Destruction. Dreams dashed in one fell swoop. Death isn't a peaceful sleep or an easy descent into old age in Keats' work. Instead, it's a particularly bitter reminder of all the things that the poet imagines he'll miss in a world that's teeming with beauty and wonders. Keats may think a lot about death, but that doesn't mean that he's comfortable with his own mortality. Luckily, he seems to enjoy dwelling in uncomfortable spaces.
By the end of the poem, the speaker feels more comfortable with the idea of his own mortality.
By the end of the poem, the speaker is even more afraid of dying.