Howdy, I may be a bit late offering my thoughts but when I saw the question and then your example I know exactly where you are coming from since we I remember this strugle many years ago when I was starting out into portrait art.
Some of the best excercises to help eliminate this is drawing from a cast figure for example or if not that then anotherone that is good is to take a ball or cylinder like a can give them a light source then paractice drawing them with the strong contrast of dark to light will greatly improve your realistic touch.
My favorite excersise that helped me acheive realism was drawing an egg and making it look real.
This forced me to have to examine the values from dark to light and even reflective lights.
Also it was mentioned to not work on all white background. I tend to disagree with this line of thinking, though I understand the thinking behind it, I just dont feel the rule should be absolute. It may be the effect you are wanting, as a matter of fact white backgrounds can really make a piece pop if done right. So its really a matter of desired effect. IN fact, many top illustrators, and some of my favorite protrait artis or figurative artist work with white backgrounds for alot of their work. It all has its place, but like anything in art it takes practice.
If I can get a moment of time to do this I might throw a couple of great excercises I do together to show what I am talking about.
MBStudios