If you really want to practice digital painting, I would use a piece where you're not as distracted by factors completely un-related to digital painting. This portrait demands that you get the likeness right, anatomy right, etc. Fabric is a toughy, too. Digital painting skill itself in my opinion takes a backseat here.
Solution is to practice on simpler objects - that way you can focus on getting your brushes right, how to manipulate the paint on the canvas, get the painted look/style you're after, etc.
Practice portraits with mere lineart sketches. You'll get more done, faster.
Those eyebrows sure are bold. Why not try a riskier, more impactful/interesting lighting set up here?