Sarah's poem is a thank-you note to an ex, and it takes a while to notice that the thanks isn't kind. On the surface the speaker is grateful: the other person left, and that taught them something. But the lesson is a sharp one. They are "entirely better off" without this person, and they deserve far more than "you could ever give me."
The speaker sounds calm, even a little smug, but the politeness feels like a weapon. The final lines turn the thank-you into a quiet insult, since the ex is told they were never enough. The short, broken lines, with "learned" sitting alone, give the poem a measured, deliberate pace, as if the speaker is choosing every word. The mood is cool, defiant and self-assured. There is hurt underneath, though. Someone fully over a person wouldn't need to write this, so the poem hints that the speaker is still working to convince themselves.