Most stubborn troubles that an artist encounters, is drawing the mouth from a side angle. It literally took me two years to accurately draw a mouth believable. My super power was observing manga comics like Naruto, Bleach, and Death Note. As an artist, exploring and observing should be used to your expense. Anyways, 1, from the side view, you should see the lip. The lip line NEVER covers the entire lip, otherwise it will look as if the person has no Philtrum ridges. A Philtrum ridge is simply the "curve" that dips between the lip and nose. The lower lip sets into the upper lip. Add a creasing division line to bring out the curve.