The overall silhouette of the skull is fine, right now the biggest issue is your lighting and some minor anatomy problems. You should try thinking about the skull in terms of a sphere, with the sides being flat as shown with sketch.
Since the lighting is coming from above, it should be creating a sort of gradient; with the jaw being the darkest
and continuing with the top of the cranium being the lightest, since it's receiving the light head-on (ahaaaa)
Would really recommend looking at references for the skull so you can pick up on the anatomy.