When drawing a person's face people usually have a preferred point of view, for example feeling that they can draw a person facing to the left better than facing right. In order to fix this, you can reflect the image now and again while you are doing the line drawing and check that you have got the balance right.
Open the Layer window and tap ① Invert Layer Horizontally.
Continue to flip back and forth, adjusting the image until it all looks OK.