The speaker starts with a piece of advice everyone has heard: it gets better. And the poem admits straight away that this is true. Things do improve. There's no bitterness about the promise itself, which makes what follows hit harder.
Then comes the turn. "They" left out the second half of the story: that life can fall apart again, and often right when happiness has finally arrived. The word "again" matters here. It suggests this isn't the first collapse, and that the speaker has been through the cycle before. The short, plain lines, with their lowercase style and broken rhythm, feel like someone speaking quietly, maybe a little tired.
The mood is honest and deflated rather than dramatic. There's no anger, just a weary kind of realism. The poem doesn't say things never get better, only that better isn't a finish line, and that the fall can feel especially cruel after the climb.