To balance it out, I choose an almost white color and select an area on her face that the light will be shining on (that would be her left side) with the Freehand Lasso Tool (L) with Feather set to 1 px (see c9).
Changing the Brush from Darken to just Normal, but still retaining the other settings, again I build up on the area until I’m satisfied with the brightness.
Most of the time I’ll be switching from adding the light reflections to shading the dark shadows, (as you see on Image c10).
Here you see the bright light shining just below her eyebrow, now I’ve selected an area of her eyelid, went back to the darker shade, and slowly build that up to match the dark areas of her face.
By implementing the same steps of selecting areas on her face and adding dark shadows and light reflections, eventually it will look like this (see c11).
There will be some incidents that you’ll end up going outside the lines, (so to speak), but that’s not a problem. For instance, the hair area will have smudges of the colors implemented on the face. This is where the original flats layer will come in handy, because you can always select the hair area there, go back to the top layer, and just refill it with the hair’s base color.