The speaker describes a day that counts as good because they managed to hide their feelings. When other people mention an ex, the speaker doesn't flinch or give anything away. But the poem is really about everything that stays unsaid. Underneath the calm surface, they still love this person, still lie awake at night, and can still feel a kiss. Even in the dark, they don't feel alone, because the memory of the other person is right there with them.
The structure makes this clear. Nearly every line begins with "I didn't," a list of small acts of restraint that builds up the pressure of what is being held back. Then the last line brings a quiet, sad twist: this is one of the "good days." That tells us how hard the other days must be. The mood is hushed and bittersweet, a mix of pride at getting through and the ache of still being stuck in love months after the breakup.