Ditch the detail and work on form. You have two dimensional shapes. The purpose of shading in the first place is to trick the eye into thinking something is three dimensional when it's really on a flat surface.
Look at your character from different angles, it might help you when figuring out how light casts itself on an object.
Here I've drawn your character's basic form, then applied it to your sprite. Think of the light as a mask over the detail. I changed the outline of your sprite based on the details (the plates) inside. The outline itself is a stylistic separator, and should in no way restrict form.
Edit: I forgot to mention, you have unnecessary shades in your palette. I used your palette, but I only used four colors. It's a common mistake to work zoomed in real close and think that some shades make it look better, when in actuality they are impossible to see far away.