You have to use highlights and shadows together, in your piece you're not only using highlights and shadows separately you're also using single lines for highlights for example the nose. This could've been done because one of two things, either you dont grasp how light and shadow works (which works best using a reference) or you are afraid of getting abity messy. Also avoid using similar colors that don't complement your artwork or show impact.
Heres my quick edit of your piece.
