At the beginning of the poem, the speaker's ride on Ariel is terrifying. She "cannot catch" the horse's neck; she's lost control. And she doesn't like it. (Can't say we blame her—this horseback ride sounds super-scary to us, too.)
Hauls me through air— (15-16)
Here, we can once again sense the speaker's fear. We see that she has no power, that she's "haul[ed]" through air by the uncontrollable Ariel.
I unpeel— Dead hands, dead stringencies.
And now I Foam to wheat, a glitter of seas. The child's cry
Melts in the wall. (17-22)
In these lines, we can see the speaker's transformation. When she "unpeel[s] / Dead hands, dead stringencies," she lets go of the things that tie her down in life. The cry of a child (or perhaps her own childlike cry of fear) melts away. She's letting loose, and she likes it.
And I Am the arrow,
The dew that flies Suicidal, at one with the drive Into the red
Eye, the cauldron of morning. (23-28)
In this final, transformative moment, the speaker embraces the fact that she's now "at one" with the crazy, galloping drive of her horse. She's submitted to the experience of the wild ride. She's a free and unencumbered woman, and she's transformed (metaphorically) into an arrow. There's a lot of deathly imagery in these final lines, but death here seems like freedom, like a new start.