It's not a mirror, it's a doorway. Yuya's face scrunches at his own reflection. Maybe if he pretends the mirror isn't there? Or imagine his room, where he should be walking out of...

This would be easier if he had more than a vague awareness of how magic feels. If only he could see it.

It takes a couple minutes of focusing, before the magic pulls inward to the mirror, drawing Yuya forward. He thinks he's pushing it into the mirror? But it might be pulling him.