Really good! The one-shot conceit stays engaging throughout thanks to the short length and all the flourish, and the mirror is also just a very clever trick for making the visual language of VNs work in what essentially amounts to a monolog (at least in the sense of the other party being a spiral of light it feels right to not center visually). The gimmick does come off as a good way to add a layer of disorientation and irreality, too, since the surface of a mirror feels like more of an "objective" medium than just seeing the events through a character's eyes directly. The audio design might deserve as much credit; I especially like how the game plays with the volume of a music, a too often forgotten way to add some variation and emphasize shifts in mood. All in all, in terms of presentation, it's a great concept pulled off with style.
And the writing is just as excellent and just as great conceptually – this certainly makes for one of the most creative ways to use the jam theme I've seen so far. The game's formal tricks help it convey information very naturally, and the character voice works. One thing I was left feeling sort of hesitant about is whether the protagonist's internal journey really lead into the resolution naturally, but to make up for that, there is plenty of catharsis in the final scene. A good read all around in a very polished package.