Just pay attention to the shades, and the things that make it really look like Amanda.
Especially the place and the angle in which you see things (the eyes, the ears, the chin, etc).
For example the eyes, do you see how they are protruding the face? You look at her eyes from an angle, so the roundness is also portrait in the way they protrude. This also affects the way you see the pupils and themselves.
Also note how shading affects the way you see the nose. On your drawing the nose is flat because it only consists of a few lines. If you add some shading you will immediately see a huge difference.
And last but not least the hair. Do you see how it curls? It curls to the outside. And there you have shading as well, it's especially dark close the her head.
That's all for now, I find it hard to explain these kind of things in English (as you may have noticed).