Like a lot of artists I have doubts about my drawing skills. So last summer I signed up for a 5 day cast drawing course at the London Atelier. They teach the ‘sight size’ method. It’s a very laborious way of drawing and accuracy is everything. I spent 4 days on the drawing of the ear (below) and could still see that it needed more work.
I could have spent an extra day improving it but had got to the point where I just didn't want to. I'd had enough of it. I was impressed with the results but I didn’t enjoy the intense and lengthy process. It was useful though and I would recommend trying it even if realism isn't what you are aiming for. There is a lot to learn that can help your drawing generally.
The tutor told me this about the method, "Anyone can learn how to draw using the ‘sight size’ method and lack of ‘talent’ is not a barrier. It just takes longer for some. It is all about close observation, detailed measurement, and meticulously following the methods taught."