The speaker feels unwanted, like a third wheel in a situation they can't escape. The word "extra" does most of the work here. At first it sounds like it might mean something exciting, but the speaker quickly rules that out: it isn't "extra like the sparks." Instead, it means surplus, something that doesn't belong. The comparisons make this clear and a little funny: a mosquito buzzing around your head at night, a dog stuck among cats, a ghost nobody notices.
The poem seems to describe a moment when "you" is with another person, and the speaker is left on the edge. The sharpest sting comes at the end. The speaker points out that "you" once admitted to feeling extra too, yet keeps making others feel that way "all the time." The mood is hurt and tired, with a dry, wry humor covering the pain. The plain, almost casual wording makes the complaint feel honest rather than dramatic.