Hey Anke, Your drawing has a lot of personality, but I think you missed the point of the exercise. You put in too many complex shapes including all those teeth. The idea is to use very simple shapes to describe the portrait, 10 or less. I did a quick sketch to show you what I mean, I hope it helps :)

I can see how tense your grip is from the second image. Take a pencil, relax your grip, relax your wrist, lift your whole arm. Barely touching the paper with the pencil try drawing by just moving your shoulder, then just moving your elbow and then just moving your wrist (all along try to be mindful of keeping relaxed). Practice with your arm not touching the table at all while drawing from the shoulder.