Let me start by saying that I’ve done a fair bit of life study. Most of what I’ve done has been in the form of charcoal portraits. I’m a little slow, but considering how short of a time I’ve been doing art (sporadically over a couple years or so), my portraits usually come out looking like the subject.
What I don’t get is sketching, and by extension cartooning/illustration. For as well as I can do a portrait or still life, I’m the worst sketch artist, and cartooning is beyond me.
For example, take Calvin and Hobbes. His head is the size of his entire body. A couple of zig-zags make up his hair. Yet, the whole thing together makes a visually coherent character.
Or, take a look at this:

(it’s not mine, I found it on deviantart)
It reminds me a little of Sprout, but anyway…
That scenery wouldn’t really look like that IRL. Instead, it’s a very abstract presentation of a reality. You can even see variety in species among the trees.
Realism, I can do. In fact, I only learned realism because I wanted to learn illustration and it was recommended that I start there. Illustration, I can’t really wrap my head around. I practice, and everything comes out looking like a pre-schooler drew it. What am I missing? Any advice?