this is a sh*tty fix but if you add a +90 to sprite angle so..
anglediff(Sprite.Angle, angle(Sprite.x, Sprite.y, Mouse.x, Mouse.y))
becomes:
anglediff(Sprite.Angle+90, angle(Sprite.x, Sprite.y, Mouse.x, Mouse.y))
Then you can have it so if this value is above 90, then it's on the left.. if it's below 90 it's on the right.