Four short lines carry a lot of hurt. The speaker is struggling with depression, and says it's hitting hard right now. They believe the only person who could ease that pain is someone who has moved on and is now with somebody else. That leaves the speaker feeling stuck: the one source of comfort is out of reach.
The last line is the sharpest. Calling themselves "just another second hand lover" suggests they feel used, secondhand, like something passed over and left behind. The word "another" repeats from the line before, where the ex is "with another," which makes the speaker seem interchangeable, one of many rather than someone special. The poem never explains how the relationship ended, so we only get this raw moment of longing and low self-worth.
The mood is bleak, lonely, and quietly bitter. The plain, simple language and the rhyme of "another" and "lover" make it feel like a confession said out loud, without any polish.