Amy's poem looks back at a single moment of happiness and treats it as both a gift and a loss. The speaker says it happened only once, but that was "enough." Yet the next lines undercut that calm acceptance. The speaker will never forget it, and describes themselves as "innocent till then," which suggests something changed afterward and that innocence went with the happiness.
The ending turns bitter and a little jokey. "He's not to blame, obviously" sounds like someone talking themselves into fairness, and the word "obviously" hints that blame is exactly what the speaker is tempted to assign. Then "Thanks brain" is sarcastic, as if the mind keeps replaying the memory against the speaker's will. Judging by the tone, the poem sits somewhere between nostalgia, hurt, and dry humor. The short, plain lines make it feel like a shrug that is hiding something sore.