Part 1. Understanding our reference.
Okay,
So I took the photograph and first wanted to illustrate how many light sources are actually on the figure.

Now I'm not saying that this is bad. This is just extremely challenging for a portrait painting. The amount of light sources that you have to achieve in a painting are at most two sometimes three.
We have a flat surface that we are trying to make the illusion of three dimensions on. The easiest way to do this is control of our painting and subject. By composing the light's contrast. The viewers eye path.
There are very few life drawing sessions that I've been to where there is a secondary light source, let alone 5.
Traditionally, (and I may have heard this theory elsewhere but in my opinion ) this limitation allows artists to control the environment and composition to allow them to play. And thus where they want you the viewer to focus and how they will express themselves through mark making.
If I only have one light source I have more control over the kinds of transitions that I have to play with (value, hue and saturation and mark making) and wether or not they have the effects that I the artist desire in my finished piece.
If I have many light sources this limits my room to play with these tools of transition of form, value color and mark making abstraction. All these take a backseat to the laws of physics I now have to follow in order to illustrate what I see to the viewer (if that is after all my intent) . Especially if I am copying for the sake of studying.
Its the difference of being able to dance through a barn door in the dancing style I want vs having to crawl through a shoulder width air vent. In one anything can happen. In the other, there is only one direction to the exit. Which one is more fun? Which one is more challenging?
Moving on.
Now lets do some editing for the sake of studying this picture. First lets make it the direction you posted your study and line it up.

If we do some simple sight size orientation for comparison we can see where the measurements were 'off' and what didn't exactly feel right.

Now this isn't how we draw! This isn't fun! This is the way we critique and find flaws in our work before we show it to anybody. lol
One way we can do this to practice sight size orientation without it being a so much of a handicap, or as shown above, a boring pain in the a**, is to just drop a line that we will erase later.
But in order to drop a line accurately to digitally study a photo we need to work within the same frame ratio so we can compare our work to the original.
Old masters chose the size of the work they made based on the spaces in which they would be permanently hung. They mathematically decided the best compositional frame ratio which also decided their best oe perhaps expressive composition for the room, not just the frame. So for studying masters compositions, and studying photographs the beefing up your sight size orientation.
Like so.

Now here comes the interesting part. Regardless of your skill we can see where you characterized the drawing. All the greats caricature even a little bit.
We can see what you decided to repress on her, what you decided to emphasize on her and what you decided to leave untouched! This can tell us about your subconscious tastes in the female form. Where you see yourself, your mother, a family member. What your left side of your brain decides is the correct symbol for a nose, chin eye and ear and takes over to replicate that symbol instead of letting your right brain take over and use the shapes in front of your eyes.
In order to shut down the left side of the brain we need an easily unrecognizable inorganic reference point. This is the dropped line.
You choose where, anywhere. I just chose the inner socket of the furthest eye here. You can put it anywhere you like.
For the sake of this critique we are going to edit this picture for or expressive needs, our understanding of form and finally our understanding of composition.
To isolate the main light on the light side, the central light, I tried a few different ways to illustrate what I mean and this is one way you can try to use to simplify a complicated light source.

I did it again to illustrate my purpose even further.

We did it. Through the magic of Photoshop we finally were able to find and show you the main light source. Your path to separate the light and the dark becomes clear. With this information we can paint more confidently. I now am understanding that crazy occlusion shadow that the floor lamp is making on her jaw, it's there in order to make a jaw line definition which was lost in the ambient light.
Part 2. Drawing and painting the head. Coming soon.