Yeah, it is hard but if you keep practicing, you with discovering your mistakes and adjusting them. At least I can see improvement when you compare this one with the one you earlier did.

The starting point is to avoid mistakes (though it is impossible) and to avoid mistakes, you have to be very careful and precise and to do that, you have to observe very.
Firstly, you have to make your ouline sketch. This ouline sketching will help you to know if you have gotten the resemblance - this will guide you and give you confidence to add shading which will give rise to forms.

Outline sketching has to do with getting the shape and curves of the model properly. Sonce you are using pen, you have to make those strokes very light that one can hardly notice there was strokes there after shading. Most mistakes are covered with shades after getting the right shape.

I will say learning to draw with pen/pencil has stages. Just keep practicing. You will get there.