Hey Chiara,
Great work on that portrait, it's got solid values and accurate lighting to show the volumes.
I'll give my two cents of constructive advice if I may, because I feel like you're showing a lot of willpower and willingness to do what it takes to improve. I love that!
In a context where we're learning, it's important to be able to paint like a sculptor. This means seeing a drawing in terms of volume rather than contour, having a good understanding of where the planes of the face are, and understanding how they catch the light. Most importantly, you need to know where the hard edges are (the "cuts"), and where the soft smooth transitions are.
Your choice of brush, i.e. a soft airbrush with a very feathered edge, is not helping you learn fast now. It's great if you want to make easy gradients, but it's not helping you make clear decisions. It's the rendering equivalent of making little scratchy line segments instead of a single bold stroke of the pen.
Pick a solid brush, round, square, does not matter for now. It just needs hard edges. Make a conscious decision to block out the planes of the face with the right values, then you can smooth out where the transitions are soft using pen pressure and color-picking. Not only will this likely improve the painterly aspect of your paintings (getting rid of that plastic/artificial feel of the airbrush), but you will learn much faster. You've totally got what it takes 
I feel like over time we get to incorporate the airbrush for specific tasks like smooth color corrections later in the process, but it's not a good base to start a render with.
Good luck, and most importantly have fun!
