Someone has gone, and the speaker is left alone with the hurt of it. The poem is only a few lines long, and most of that space is taken up by repetition. "You left me all alone here" appears twice, almost word for word, as if the speaker keeps circling back to the same moment and can't move past it. In between, "You left, and walked away" adds a quieter, more final detail: the other person simply went, with no mention of a fight or an explanation.
The turn comes in the last two lines. "But it's fine" sounds like an attempt to shrug it off, yet "I'm used to pain" undercuts that. The speaker isn't really fine; they've just learned to put up with being hurt, and the poem hints that this isn't the first time. The mood is bare and sad, with a flat, tired kind of resignation. The plain wording makes the pain feel honest rather than dramatic.