I'm not experienced with shading lips, least of all realism. And I've fallen behind with my shading skills, so I can't say I'm all too knowledgable. But I can say that, for a start, your line art sketches of the lips don't look accurate. There's no shame in tracing a photo if it's for exercise purposes and you're able to credit the owner (but if not it's no big deal). Marc Brunet describes tracing as like training wheels on a bicycle. Technically we don't need to trace, but it does speed up improvement.
Second, whenever I try to paint something from reference, I break it down into soft shades (gradients) and hard shades (cell shades). And with painting anything, step one is to get the dimensions correct or else it'll never look right. Once you have that, concentrate on mimicking the shades as closely as possible. I personally find it helpful to use eyedropper tool on my reference, just so I know where the color is on the spectrum. Sometimes I also remove all saturation from my reference and painting so that everything is in greyscale. It's simpler that way. Then once you're done painting, use Gradient Map to bring back color, and then touch it up a bit if necessary.
As for lip shapes, lips are one of those tricky shapes on humans like hands or feet. But like everything, studying a lot of it will help. I personally wouldn't try mapping out a wire frame unless I'm creating a 3D model. But finding good reference always helps. Also trace it if necessary so you have a benchmark of what it should look like. Then try to draw it yourself. Don't forget to add teeth and tongue when necessary. Trust me, they make a big difference.
I hope I was of some help.
