Good work! The structure looks ok but I think it looks like you're shading is a bit uneven. Try to break the shadows down into large shadow shapes of consistent values, then add the detail. Are the darkest shadows in the right place? Look also at soft and hard edges. The side of her face in front of shadow could merge a little with the background and still look good, it doesn't need to be a hard line of separation. It looks like it might just be you drew the face contours first, in which case I'd suggest using a lighter lead so the lines don't dominate the drawing.

I think this drawing is beautiful and well-simplified! Good idea to study from the masters. I notice on her forehead, there's a line that seems to go straight down the middle or side of her nose. I don't know if you're using that line to place the features, but if you are, it might be helpful to use curved lines that follow the long curve of the head's oval (or the short curve, if you're placing the eyes). Very nicely done! I especially like how you've concentrated the darkest values on her face, making her stand out from the background.