Carlos, I find the structure very solid and convincing but have reservations on the way you then developed the painting.
This is something I tell myself when painting portraits: so ok, you have painted her hair brown, her dress black, her skin pink, the background ochre. Do they integrate?
I think in your work ,this time, they are all very separated, mainly in tone. The hair is very dark but the skin is very light, the black is dark again (look at Sargent for light blacks or at
this painting against her skin, but it should be lit by the same light .
Did you work each section at a time? I have found very useful for me to work in and out the same edge, and find out if there are parts in which that edge might disappear. try to view your photo in B&W and this will appear more clear.
Also I find that when the whole of the background is painted in, the figure should be painted all the way to the bottom edge, even if it is sketched roughly, to avoid that floaty feeling.
Now that you have establish so well her proportions, could you do one more sitting to work on unifying the tone or letting some of the colours bleed into another area?
Ilaria