You're off to a fantastic start, and it's great that you're willing to work from reference! Here's my feedback:
Don't be afraid to use hard shadows! This is the drawback of using fashion photos as reference, since they're often very heavily airbrushed to eliminate anything sharp (and imitating that is both very hard and not helpful before you're at semi-pro level IMO). As it is, the fuzzy shadows don't help form the 3D shape in your drawing, especially in the hair. Think of the hair in chunks and shade those chunks first, then go into detail. Also think about what is causing the shadow. Is it soft light (generally softer transitions) or hard light (generally harsher transitions)? Is it the result of a curving away from the light like a ball (generally softer) or is there a sharper edge (sharper shadow)?
Also try to use tinted shadows and highlights, cool shadows (=blueish, reflected from the sky) and warm lights (=orangeish, from the sunlight) are popular and comfortable to the eye.
Speaking of shadows and light, make sure to have a uniform light source! The shirt seems to be lit from the right, but the neck from the left.
Feel free to ask me if something doesn't make sense!