I can't speak to your technical ability but I do have one suggestion for approaching digital art: You need a different brush, with a lower stabilizer. I can tell that you have skill, but your current brush isn't working with you. I personally use the "Pencil-5_Tilted" brush that comes with Krita (a fantastic free program) because it allows for sketchier styles that use a lot of quick strokes (like yours!).
The pictures you've posted remind me of when I used to try sketching in Paint Tool SAI with the stabilizer a little too high, which causes the brushstrokes to be much stiffer than you intend, often with strange little curls on the ends. If you look at the shading on the cheeks of your portraits, you'll see what I mean by this. Again, this isn't anything you're doing wrong, but rather just a matter of an incompatible brush for what you're attempting to do.
