There are some good and bad things about this piece.
For a portrait, your rendition of skin is quite decent. The shading around the lips and the light on the cheeks is very nice. You have used the good tonal range to display what you wanted to show, and the facial anatomy is acceptable.
However, many things hinder the realism of this painting. For starters, there are no shadows at all; her hair does not cast any shadow on her skin, and does not get darker as it gets away from the light (such as behind her shoulders). The brush you used for the skin is appropriate, but you should have cleaned up the wobbly contour line in her face.
The rest consists mostly of local touch ups. Her eyes are not dark enough (the middle of the eye should be a lot darker and the eye area should have more contrast. Make sure the eye-liner is equally dark on both eyes! In addition, her entire face could use more contrast (darker nostrils, harder lips, etc.), but you could fix that with an adjustment layer.
The hair could be inproved if you used a much smaller, uniform hard brush and painted a LOT more individual hair strands. A trick I use a lot is to paint several (8-10) individual hairs and copy paste them to save time! Make sure you warp and edit them every time or else it becomes too obvious the pattern repeats =P
Finally, try to spend as much time on the accessories as on the rest of the drawing; the items in her hair lack definition, shadows and shape.
If you keep practicing and make sure you fix those mistakes with every new painting you make, you'll achieve great results! Keep drawing!
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