A letter arrives and tells the speaker to go. The poem never says who wrote it or why, but the title, "Mismatch," suggests a relationship that has ended because the two people didn't suit each other. What matters here is how the speaker responds.
The speaker doesn't argue or break down. "I go on / making myself fit" plays on two meanings of "go": leaving, and carrying on. Both happen at once. The speaker leaves the relationship and keeps living, but the effort shows. Fitting oneself into "the midnight clock" is a strange, vivid image. Midnight is the moment between one day and the next, and it is also a dark, lonely hour. The speaker seems to be squeezing into a narrow space of time, perhaps sleepless, perhaps simply adjusting to a life that has shrunk.
The mood is quiet, tired, and restrained. The irony is that after being told they don't fit, the speaker keeps trying to fit somewhere else.