Looking back on a relationship, the speaker realizes it was never what it seemed. At first, she loved him completely. The line about him hanging her moon and stars shows how he seemed to be the centre of her world, and the one who made her feel she "meant something." That small phrase suggests she may not have felt very sure of her own worth before.
Then the poem turns on the words "But now I know." She has learned his "true intentions," and the title, "An Experiment," tells us what she found: she was something he was trying out, not someone he cared for. The final line, "And I failed," is the sharpest moment. Even though she was the one used, she blames herself, as if she had been taking a test she never agreed to sit.
The mood is quiet, hurt, and raw. The short, plain lines feel like someone stating a painful fact because she has run out of things to say.