The poem is just two lines, but it holds a whole lopsided relationship. "11:11" is the moment people treat as lucky, when you pause, close your eyes, and make a wish. To say "You were my 11:11" is to say this person was the thing the speaker wished for, or the small bit of hope they reached for when things felt uncertain.
Then comes the turn: "But I wasn't yours." The speaker wasn't wished on in return. The feeling was never shared, and the speaker has understood that, probably after the fact.
The mood is quiet and wistful, with a sting of hurt underneath. There's no anger and no big explanation. The short lines and plain wording make the loss feel understated, as if the speaker is stating a simple fact they've had to accept. That restraint is what makes it land: it sounds like someone saying something painful as gently as they can.