This was very helpful! I have never thought about the fact that lips follow the curves of the head. Suddenly 3/4 view lips made sense. I struggled a lot with that when making my anime illustrations, so I ended up dropping 3/4 view all together. Definitely going to give it another try the next time a draw humans :D
Thanks, @pixielolz. Knowing that one thing about the curve of the face helped me enormously. I couldn't figure out the problem for the longest time. Then I looked at the skull and it made sense.