The speaker holds on to every detail of a relationship, from the first kiss to the best Christmas, and discovers that the other person has forgotten some of it. That discovery hurts more than anything, because it makes the speaker wonder whether those moments ever meant as much to the other person. The memories rush in "like a wave during a flood," yet they feel worthless if only one person keeps them.
The speaker responds by shutting down. They say they've grown "too strong" to cry, and they sit with a straight, blank face, neither smiling nor frowning. They suggest this kind of hurt has happened before, which makes the numbness feel learned rather than new. The mood is quiet, tired, and guarded, with grief held tightly under the surface. The closing lines turn the pain into a bitter conclusion: people who no longer care are often the ones who once cared the most, and the speaker seems to be describing their own heart.