The speaker is missing someone and has turned that longing into a habit. At first they study the other person's face closely, whether the person is asleep, talking or eating, so they can call it up later when they're alone. Closing their eyes brings back a smile, and the memory feels like a comfort, something to rely on "when I would be out of air."
Then the comfort sours. The speaker realizes the person can only be reached in imagination and dreams, and that makes the memories hurt, so they try to erase them, even the smell and the small details. The last stanza admits this has barely worked. Days of trying to forget end with the same image, the face appearing just as they fall asleep.
The mood is tender but worn down, with a hint of frustration in the casual "pretty damn well." The speaker's own effort to remember has made forgetting impossible.