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I deeply enjoyed this extended metaphor throughout the piece. Your description of the backbone is so beautifully and thoughtfully written, that one almost believes that the "you" you're referring too, actually is your backbone. However, this piece also has a simplicity factor, because it is short and keeps the lines "to the point" which also adds to how brilliant this is.
If you were to add detail, perhaps just sensory details? I feel as though you could add more on the "feel/touch" sense and apply it to this piece. Like, comparing how it feels or what it makes you feel. If you were to expand, you could add something about how "you've been there since the beginning" since it is your backbone and you've had it since "birth".
Great work! Keep writing! (Sorry this review is so late!)