You are getting better!
I think I can help you with some tips:
I think, that your main mistake, is that you tend to "stereotype" what you are drawing: When you draw the mouths, you use your memory to draw them, not your eyes You draw "perfect" lips. Take a closer look at someone's lips. You'll realise these are irregular and "ugly" xD. They are not as cartoons. Try too detect the diferences between two different people's lips&eyes&noses.
Another tip, is to draw the same drawing over and over again. I think you should keep up with spheres and cubes for a while. Then, spend some time on eyes. Try like drawing a dozen eyes before you move on to noses.
My last tip: dont draw each teeth individually. These make the mouth look a monster Try to draw them as if they where the same shape, instead of different tooth.
Keep it up!
"For a man who doesn't know to which port we wants to sail to, all winds are unfavourable"