Hey op, it's ok.
You haven't improved in a while, which causes frustration, that's normal. I'd go as far as say it's a part of the process.
What you need is learning how to learn before you learn how to draw.
Don't worry about the human body or anything else.
Let's say you want to learn how to draw a duck, in any position, without a reference.
Set up a tilted surface, a piece of paper and a pencil.
1. Learn to draw a dot.
2. Then a line.
3. Then a shape.
4. Then a form.
5. Then, you have to learn what a duck looks like.
Not just "oh it's a bird with two legs", but more like "this is what its skeleton looks like, this is how its joints move, this is where the muscles attach, this is how it walks..."
Now when you want to draw something else, you'll only have to repeat only the last step, that's what we call expanding your visual library, and it never really ends.
This is a summary, your battle plan.
To draw a line properly, you'll have to practice every day. To construct a form properly, you'll have to learn perspective, gesture and render it after wards. Then there is composition, color theory etc, but really, there's nothing more than those 5 simple steps.
Draw every day, be precise, seek advice and criticism, look for inspiration, listen to podcasts and video lectures, read e-books, try out new tools, analyze your mistakes and most importantly, enjoy the process.
We're on our way to draw cool pictures and get paid for it, how fucking cool is that.