As others have said, increasing the contrast by adding deeper shadows would help, however I suspect there is something else that could help break away from the "cartoony" feel that you're seeing. Our brains are masters of creating patterns in and simplifying what what we see. This often comes into conflict when drawing because there is a disconnect in what the brain remembers about how features should be shaped verses what our eyes are actually seeing. For example: In the reference image you can't actually make out the majority of the eye. There is light hitting part of the upper and lower lids, but everything else is completely in shadow, including the white of the eye. There is the impulse to overcompensate and define the whole eye, even if we can't see it because the brain knows how eyes "should" look based off past experience. Another example is the placement on the collar. We have a tendency to put them too low because when "know" it rests on the base of the neck. In the photo, however, the collar has a lot of bulk and covers the majority of the back of the neck. Training yourself to recognize when your brain is filling in info based on your backlog patterns and symbols and instead drawing only what you see, is what what my college art professors called "using you artist eyes." I hope I explained that in a clear way. It was a concept that took me a while to grasp but helped me immensely when I do realistic portraiture.