i do not recomment using mirror because i had the same situation as you and it didnt work.so i used two different animations already mirrored in editor.
edit:now i know how to use mirror.
1.set not mirrored for the direction of the current animation of the sprite.
2.but set mirrored for the other direction.
eg. if the sprite is facing right for the current animation,then set not mirrored in events when he moves right. and mirror for left movement.