The speaker looks back on a relationship she once wanted, but now sees that her former partner was not the person she imagined. She feels relief and confidence rather than heartbreak: the breakup has helped her recognize that leaving was fortunate. Her tone is celebratory, though also pointed, as she imagines the other person regretting the loss.
“What goes around, comes back around” suggests that people eventually face consequences for how they treat others. “Dodged the bullet” is an idiom meaning she narrowly avoided serious trouble; here, the failed relationship is presented as something she escaped. Calling the ex “the best thing I never had” turns a familiar romantic idea inside out: not getting the person was better than having them.
The lyrics mix genuine relief with a wish to make the ex feel rejected in return. Lines about not caring and being glad the ex is hurt may be bravado, or may show that some anger remains. The speaker says she is “so over” the relationship, but repeatedly addresses the former partner and imagines their regret. This leaves room for an interpretation in which moving on is real, while emotional closure is not entirely complete.