Hi Sarah, I have been wanting to comment on your portraiture work for a while now but just run out of time...your work is really great and see a lot of methods I used while learning portraits.
There are key areas that must be talked about your portraits that in time will let you create even more astounding portraits...
Proportions is a key element when doing portraits, the eyes, nose and mouth area are the most crucial, such as the eyes for the most part in all portraits should be the same width and also have that same width spaced between the eyes...also the pupils of the eyes should line up with the corners of the mouth. The eyes in this piece are a bit to far apart but works well for this piece, just something to look for in your next piece.
I really like the highlight and shine on the nose and the cheeks, brings out the piece and your fold in the shirt is done rather nicely.
Here is what I would suggest, while learning the portrait, take a little more time to develop the outline and double check proportions before laying in any shading. After you have the outline sketched out hold that sketch up to a mirror and look at it while it is reversed, it should point to a lot of things that are not quite right and you will be able to adjust them before you start in with the shading.
I really like your portraiture work and look forward to seeing you grow as an artist, keep drawing and learning from each piece.