Gigi's poem sets two versions of the speaker side by side: the child she was and the teenager she is now. The second half answers the first almost line for line. Days that were long are now too short. Parents who were giants have become trolls. Boys lose their cooties, and the speaker turns into the moody teen she once noticed in others. Even reading flips, from a hated chore into an escape.
The tone is light and a little jokey, with details like naps and cooties, but sadness runs underneath. Friendships are now "lies," imagination is rarely used, and games have mostly dropped away. The speaker seems to miss the playfulness she has lost.
The ending is the most telling part. A few things stay the same, like laughing at silly jokes and lying, but the clearest constant is the last line: "no one understands me." Her loneliness hasn't changed. It has only grown up with her, which leaves the poem funny, wistful, and quietly lonely.