The speaker, writing as part of a "we," looks back at a friend who was struggling while everyone around her saw only a cheerful performer. At first the group defends itself: they didn't notice, they couldn't have known, how could they have helped? The images of "best actress" and a smile "like butter on toast" melting away suggest she hid her pain behind something that looked warm and ordinary.
But the excuses start to crack. In private, the group remembers the signs: her lips turned down, a tear explained away as hay fever, a friend who asked for advice and never asked how she felt. The title's claim that they "couldn't have helped" begins to sound like something they repeat to feel better.
The final lines turn the denial around: "We all noticed she had changed." The mood is guilty, regretful and quietly accusing, with the shift to "you" making the blame feel personal.