Four short lines carry a whole heartbreak. The speaker calls herself "just a girl," which sounds modest, almost like a shrug. She seems to be saying she isn't anyone special or powerful, only a person who loved someone and couldn't fix what was wrong. That small word "just" makes her sound tired and a little helpless.
The real weight falls on the last two lines. The boy "didn't / Love himself," and the line break leaves "Didn't" hanging alone for a moment, as if the poem is holding its breath before the blow. The poem suggests the speaker's love wasn't enough, because he couldn't accept it or believe he deserved it. There's no anger here, though. The tone is quiet, sad, and a bit resigned, with a sense of understanding mixed with hurt. The poem doesn't explain what happened or who he was, and that gap lets the feeling land. Many readers will recognize loving someone who can't take in that love.