Imagine trying to write about someone who hurt you and not knowing where to begin. The speaker asks what they would even say about "the one," and the answer turns out to be a kind of apology. Not to the ex, but to everyone who comes afterward.
The speaker admits they let themselves be hurt, and now that pain follows them into every new relationship. When someone says "I love you," they hear the old voice and see the old face instead. That memory "clouds" them and makes them doubt what's being offered. The "after taste" in the opening lines is a good image: something bitter that lingers long after the meal is over.
The mood is sad, regretful, and a little tired. The last line is the quietest and the most painful. Because the speaker once believed so completely in one person, they can't quite believe in anyone else.