Don't edit form, just get better at simplifying. Average your transition values, then add your high-value points. It's a lot easier than editing low-value points over and over. That's how the masters did it. Building up.
There are two ways to render - airbrushy, and hard edges. Know when to use both. Make conscious decisions where you want to smudge and where you want to keep your planes sharp, because edge sharpness affects values as well. Choose your big gradients the same way you choose your big edges.
Break your image into less than 10 swatches. Then mix in and out of them. Oil painters have to mix everything by hand, so they simplify the palette as much as possible. Trouble is, in digital colours don't mix so smoothly....hmmm..
Photoshop is a big distraction, because it has so many tools and options. Paint as efficiently as possible. Don't get lost in editing. Do it all in one stroke.
Keep your painting mostly dark, then add one area (no more than a sixth of the painting) where all the bright values are. eg. Brom.
Thanks for your advice, Josh. I'm still really bad at it, but hopefully it'll help. :D