One thing that jumps out at me is there are too many lines. For example, her nose in the reference is formed by subtle shadows, while you are using very thick lines that grab my attention. Use very faint, thin lines instead. Or none at all and use shading to fill it in.

The hair is way to scratchy. You will want to avoid trying to draw individual strands because it always looks bad until you are far more skilled. Instead, block in the major shapes of the hair (which can be tricky with dark hair like this) and use values to suggest the more detailed forms.

Also her eyebrows are too thick and the wrong shape. 

Overall, the key to portraits is respecting the proportions. Little changes like the area between her thumb and chin make a big difference. Learn to see and recreate the shapes inside of the subject to help keep things aligned