I'll just second the others' observations and say also that we should all be so lucky as to reach an "impasse" that reflects the level of artistic intuition and understanding of technique that is demonstrated in your work. If the learning curves of any others here have been a smooth constant ascent without any plateaus (or potholes or slippery slopes), they're pretty rare amongst the usual experience of battlers in the cause.
A very small technical note, relating to the eyes. I don't know if this "means" anything, but the light on the subject is coming from our upper left, but the catchlights in the eyes are in the right side of the corneal area. The catchlights are also quite large, as well, and the combined effect is to kind of make the eyeball "bulge" out rather sharply, especially in the eye on our right. The shadow under the brow on that eye on our right might also extend just a little farther from the nose shadow -- not all the way across the upper lid, but perhaps into the area where the upper lid is meeting the iris. This would allow that area to recede a little, to be "tucked into" the shadow from the bridge of the nose, and thus enhance the roundness of the eye.
As for training in isolation, it certainly has its drawbacks, and even well-written books are kind of quiet companions. I have profited greatly from art instruction videos, which besides often (alas, not always) conveying useful information, kind of invite a like-minded friend into your studio for a bit of human contact (though not, unless you want to ruin the tape, a shared cup of tea). Most of my videos are on oils, but for drawing and pastels I would highly recommend Daniel Greene's "Portrait Drawing" and "Portrait in Pastels" videos. Those are worth owning, because you'll watch them over and over. Because "new" purchase prices can be a bit over the top, be sure to check availability at libraries (including any distant libraries that may have network or reciprocal lending arrangements with your local branch). Alternatively, you might investigate online sites, such as
www.art-video.com , where after payment of an annual membership fee, instructional videos can be rented for a fraction of the purchase price (and the rental may apply to purchase, if you decide that you'd really like to own the tape, after all.) Anyway, I mention the videos because I've always been someone who benefitted more from a five-minute demonstration of technique than a five-chapter explication of it (too often read in the after-fatigue, pre-dream final stages of the day).
To the extent that you are using books, keep in mind Harley Brown's advice regarding his, that it shouldn't be perused in an easy chair, but should be open right next to the easel as you actually do the technique exercises. I have to admit I don't often do this, though I had fantastic results when I did so with Paul Leveille's oil portrait book, and he has a similar pastel book,
"Painting Expressive Pastel Portraits," with lots of procedural and colour demonstrations (though perhaps unfortunately without shots of the live or photographic reference).
Best wishes,
Steven