You have to go back to Macalania Forest and charge it at the sparkly road that leads over the trees. You can't go straight back, though. There is this small quest with a mother and a boy who's looking for her husband. When you've found him, you have to find the boy (cause he's ran off).
The boy will be standing on a crossroad on that particular sparkly path and you go past him to the place where you charge the mirror. This is also the place where you will charge your celestial weapons.