I would like to offer some, hopefully, constructive criticisms.
* Her forehead needs more definition. Put your hand over your own forehead and notice how rounded it actually is. In the drawing it looks too flat. You need a tad bit more shadow as the form turns.
* The corners of the mouth should always be darker than the rest. You have the same value across the whole split in the lips.
* Think of the whole mouth area as a spherical shape. Sublte value changes towards the cheeks should help giving it a little more volume.
* Some of the outlines are too sharp, especially on the left side(her right). I suggest letting her hair frame the face.
* Outlines are too flat. They only accentuate contour. Good outlines describe form. Try googling "lost and found lines", and look at artwork from Ingress.
* Her nose could use a little more form shadow. Especially at the nose bridge.
* Put some more shadow around the temples. Not necessarily darker, but a larger area in shadow. Look at the photo for reference and pay attention to the zygomatic arch, especially how soft and round the shape is on her.
* Her neck could use a little more form shadow. Just a subtle touch since the point is not to draw attention there. Also put a little suggestion of her clavicles.
As a final note I think you've chosen a very difficult subject. The light is so flat and comes from every direction. It's similar, though not as bad, as making drawings from model photos - which forces you to squint until you get a headache. To make it a little easier you can try blurring the photo slightly.
I hope this was helpful to you.