The speaker is ending a relationship, but the poem quickly suggests there is little left to end. They say they let the other person go before that person even thought of leaving, and that it makes no difference whether the person is beside them or not. The key realization comes in the middle: the speaker may have been the one to walk away, but the other person had emotionally left long before. Leaving is almost a formality.
There is also a note of self-doubt. The speaker believed they could trust their own judgment in giving their heart, and admits that was a mistake. The last stanza pushes the numbness further. Good and bad, fun and sad all blur together, because nothing feels different anymore.
The mood is flat, tired, and quietly sad. The broken punctuation, with its stray dashes and trailing dots, makes the lines feel hesitant, as if the speaker is trailing off mid-thought. Instead of anger, the poem leaves us with emptiness.