Portrait painting is not easy in the beginning. I know very well. There are many pitfalls, which you must overcome in the learning process. But where to start?
First it is important to name the problems.
Here I give you my top 10 error list that I published earlier. ( And I can make the list longer easily.)
1. Bad photography of the model/bad copies.
2. Poor quality of the canvas, hardly prepared with primer.
3. Wrong brushes. (worn-out or to small)
4. Inaccurate underpainting/drawing.
5. Scanty use of paint.
6. Flesh-tones too light.
7. Fear for the shadow-tones.
8. Inability to see large planes.
9. To much hard edges and transitions. (the eyes, mouth and hairline)
10. Mess on the palette.
The first four can be avoided by better preparation. The rest can be trained relatively easy.
Once mastered, the real learning can begin.