The speaker is mourning a lost relationship, and coffee becomes the way she carries it. After the breakup, she wakes up and drinks it, and the bitterness feels like the harsh words she said the night before. She doesn't love it or hate it at first. She keeps drinking it because it helps, much as the person she lost once did. It keeps her awake, it is the first thing she thinks of in the morning, and its smell reminds her of him.
The ending shifts the tone. She still drinks coffee, but it no longer hurts. The final lines turn the bitter drink sweet: having him was "the sweetest sugar," and the aftertaste lasts. The poem moves from raw regret toward acceptance, and it suggests that the pain has faded while the good memory remains. The mood is quiet, reflective, and gently hopeful, with a trace of lingering longing.