Re: New portrait - c&c welcome!
I'm no expert, but I am teaching myself portraiture right now as well, so I will be as helpful as possible!
I'll start with the positive critiques, cause I think you have some good results on some of the hardest things (in my opinion) already.
The skin tone is actually pretty good. I would actually say, the colors through the entire painting feel pretty realistic to me. Value changes are pretty good as well, meaning the difference of light/dark between the bright parts of the hair vs the dark, and the highlights of the face vs the shadows. I would also say that the proportions are not bad either. They read as realistic.
Couple of places I think you could practice/push:
Color variation; When I look at the 4 major areas of the painting (Skin, Hair, Shirt, Background). They all seem to have only 2 tones. So your skin has the pale highlight, and the reddish shadow. The hair is blonde and brown. The more variation you put in these color ranges, the more realistic it will read. If you check out reference photos of people with light hair, they have the very blonde portion, some shadows underneath are almost black, with several different shades of browns in between. Same with skin, the more you work in ranges between your darkest area and lightest area, the more realistic it will seem. The shirt is almost straight black, it would help to add very subtle dark grey details on it. You can indicate where the light is coming from even on a black shirt. (usually the tops of the shoulders will have a mild highlight on them).
Edges. This one is very hard, and takes lots of practice. You want to try to make soft transitions in most places, where you are not specifically trying to show a hard edge. So the model's left side of her face, is right against a dark area behind, so the edge that is alright in my opinion. However, the left side of her neck is curving back into that shadow, so you want a soft edge/transition there. The neck looks a bit narrow, so if you were to widen it out with a soft transition from the color it is, to the darkness of the shadow behind, it might help. Same thing with the edge of the hair on the model's right side of her face. Where the edge of the hairline meets the face, if you blend between the color of the skin and the color of the hair, and make where one starts and the other ends a bit ambiguous, it will read better.
Final thing I will add is the eyebrows. You want to have the same ambiguous edge on the eyebrows. The edges of them should blend almost invisibly into the skin tone. Then, you go back over the top of the area with some dark, thin lines to define the shape of the hair of the eyebrow.
Hope some of the above helps!
"Mais que veux tu?" - Vincent van Gogh