There are two things that have really stood in my way of drawing for most of my life and have been my greatest challenges in my continuing efforts to teach myself to draw.
The first is simple the ability to perceive my scene or object in three dimensions. For the longest time, that simply eluded me. I'd draw a 2D outline of my concept with no clear sense of how it related to anything else on the page or how to show that it had form and volume instead of just being a flat image.
The second is a fear of failure. That if I put pencil to paper and didn't produce a masterpiece on my first go, that it would indicate that there's something wrong with me and I should just give up drawing and become an accountant.
My struggle against these represents the majority of my attempts to learn how to draw. Giving myself permission to create poorly. Learning to feel my way through a drawing to be able to visualize it in three dimensions and see in my head what I am working with on the paper. As I draw more and more, these things grow easier and easier to overcome.
Since I've figured out something about shading an object in 3D recently, I'm moving over to trying to figure out how to draw human figures. I've never done anything but draw a human facing straight forward before, so drawing one from the side or at an angle is quite a challenge for me. I'm also working on drawing faces at different scales, pointed in different directions, just to try to understand how that works.
In many ways, it's like I'm reliving being a child, since I never got to draw as a kid. An 8 year old might be able to draw better than me, but hell... Where most people started drawing at 5, I started at 27. I've got quite a bit of catching up to do.