Just a quick 2c. I'm not a portrait guy really, but a couple of things I noticed:
- You've given her small nostrils. In the photos her nostrils are a lot more exaggerated. Larger and wider.
- A common feature of the references is the large lips with the down-turn at the corners. I think you need to make the top lip fuller and curl the bottom lip up into the top lip earlier, to make the bottom lip a bit more narrow and curvy.
- The lips are one of her defining features, so where you go into the shaded part of the face you could carry on the shape rather than losing definition to the shadow. This could either be by using another shade or by suggesting an outline for a few pixels, to give the viewer something to grasp hold of.
In general terms, she's beautiful but she's been made hard by the horrors of her life. Tighten up the musculature so she looks toned. Change the expression from "worry" to "business-like"
Finally, the hair gives me the impression of being something like dreads, which isn't the right look. Her hair is pretty wispy...
This is all pretty general but I don't have loads of time. Good luck, I believe you're on the right track.
