"I did not reach Thee" embellishes its overarching extended metaphor with flickers of personification, making an old trope—life as a journey—feel fresh and strange.
In the central extended metaphor, the speaker’s journey is life itself: a long, arduous, and ultimately rewarding trek. The speaker's adventure across deserts, hills, rivers, and seas can be read as an image of life's long, strange trip, which inevitably crosses deserts of loneliness, rivers of tears, hills of difficulty. Those with faith, though, can take courage from the thought that they're heading toward the beloved "Whole": God, with whom they'll rest after death.
This is a classic metaphor; Dante and Bunyan are only a couple of the writers who famously used it before Dickinson did. As ever, though, Dickinson makes it her own. It's the details within this metaphor that make the poem lively, strange, and distinct.
Consider, for instance, the moment when the speaker personifies their own feet:
The Sea comes last—Step merry feet
So short we have to go
To play together we are prone
But we must labor now
The last shall be the lightest load
That we have had to draw
This could just be read as a "feet, don't fail me now!" moment, in which the speaker is encouraging themselves more than anything—if it weren't for the fact that the speaker goes on behaving as if their feet are their companion on the journey throughout the rest of the poem. After this passage, they're no longer an "I," but a "we."
Personified, the feet become companions; it's as if the speaker recognizes their body as a separate being with a shared goal. There's a sweetness in this relationship, too. Though the speaker and their feet have trekked over endless sands together, the speaker thinks of their feet as playmates, cheerful friends with whom they’ve had some good times.
On their personified feet, the speaker travels through a personified world. The "Sun" is a "he" here, traveling a new and "crooked" path as the speaker makes their way into another hemisphere—an image that hints that the sun offers a kind of personal encouragement, showing by its new bend that the speaker is on the right course.
Last but not least, and not for the first time in Dickinson, Death itself is a person here. As the speaker at last makes the last few reverent steps toward the "Whole," Death—who has apparently been there all along—springs out in front of them and "usurps" their place in line, stealing the first look at the sight the speaker has quested their whole life to see. Of course, considering the extended metaphor, Death might have to get out in front of the speaker in order for their journey to reach its end: to meet God face to face, one must die.
Again, there’s a strange sweetness here. Death isn't some serious robed reaper here, but something more like an eager kid. In this speaker's animate world, all is hope and excitement—even if life’s journey is long and hard.