The speaker is watching someone they love be happy with another person, and trying hard to be gracious about it. They keep repeating that this is "fine," "good," even "amazing." But the repetition feels like a script they are reading to convince themselves, and it quickly cracks.
In the middle, the poem shifts to a run of "i still wish" lines. Each one is a small, specific longing: to be enjoyed, to meet the family, to be the person who holds them when they cry. The last of these, "i still wish i could be your one," is the most direct and the most exposed. Then the speaker returns to "fine, good, amazing," which now sounds hollow.
The ending compares the pain to a "law sanctioned bullet," a harm that is allowed and even approved of. The speaker has no right to complain, and being reasonable doesn't make it hurt less. The mood is quiet, restrained and sad, with hurt pressing under polite words.