Some things that flattens it is the dark cheekbone shadow (in the middle of the main shape), the eye pointing at the viewer and the gradients that pretty much stops at the same value regardless of where it is on the overall shape.
Consider the 'mother shape' more when you pick a value.
The sphere in the post above illustrates this, however, it goes to white, which probably will be bad for your color identity (which works pretty well on your painting).
It's better to use shadows to describe shapes than to keep pushing hilites, I think.
Speculars can be used to, and remember even dull surfaces look a little different from different angles. You often get a hilite where the 'normal' of the plane allows a bounce from the lightsource. Sometimes edge planes will be a bit darker even if you think the lightsource should light it.
I've made a habit of using blue speculars on planes pointing up, to suggest a sky reflecting.
Jamen jag tror att han skäms, och har gömt sig. Vårt universum det är en av dom otaliga spermasatser som Herren i sin självhärliga ensamhet har runkat fram för å besudla intet.