Grace's poem is a confession from someone who has spent years shielding another person, probably a younger sibling, though the poem never says so. The speaker looks back on a hard past, gave up their own youth, and learned early to hide their tears. All of it was meant to keep "her" innocent, and the speaker believes it worked.
But the poem is just as much about what protecting her cost. The speaker admits to becoming angry and jealous of the very person they defended, who got the safe, unaware life the speaker never had. They call themselves a monster and describe feeling trapped, with "no key to this locked door." Even so, they know none of it was her fault.
The mood is sorrowful and tense, pulled between pride and shame. The lines about "while I'm gone" hint at a goodbye, and the final image of darkness arriving in a once-starry world ends on a quiet apology.