By the end of the poem, the speaker has stopped struggling against Ariel's crazy gallop, and she becomes "at one" with the horse. She embraces this experience, and she casts herself as "the arrow" heading "suicidal" toward the red rising sun. It's an intense and visceral image, and we feel like we're right there with the speaker, atop Ariel, heading into the bright red unknown. It's a moment of death, but also of rebirth: the speaker may have shed parts of herself, but she's become a powerful force of nature.
Lines 26-28: The speaker calls herself "the arrow," and thus imagines herself in a directed flight. Ariel has given her power and direction. By submitting to Ariel's power, she gains her own power.
Lines 29-31: The speaker imagines that, as an arrow, she heads "Into the sun, the "red / Eye, the cauldron of morning." This arrow is heading for a deep red bullseye; we imagine that it's gonna pierce that baby and show it whose boss. Or is it? That red cauldron is a pretty deathly image we think. These lines give us two forces colliding.