Perspective. Composition. Color Theory.
Those are the three areas of fundamentals that cover everything when it comes right down to it. But anatomy kind of falls under all three. There are multiple books on each subject., and online courses. I recommend taking courses. But its true that it certainly takes time. Once you understand and learn these theories implementing them and critiquing your own work like a teacher is when you will really start to see a difference.
Here are two practice milestones you can start with.
Draw 100 perfect cubes. Seriously perfect. Freehand.
Draw a figure for a 1.5 hour sitting and save that drawing. Practice short poses increasing time over about 3 or 4 months till you get back to doing a 1.5 hour figure sitting. Compare the two.
Now you can pick an anatomy construction technique to learn like the Frank Reilly method, or Loomis or Bridgeman or Hogarth, who cares, just pick one you like and try to memorize it.
It will take you a long way.
I guess like the anatomy practices above, make some timed practices where you are only working on one principle or theory at a time until you get it. Dumb it down, use black and white only, or 4 grays. But Studying scenes from your favorite movies is a good place to start. Studying from golden age illustration masters even better. But you will notice a pattern of how to arrange elements for the viewer, and how universal it is.
A 5 minute study will look much different than the 1 hour study. But can you capture the essence in 5 minutes? No? try it twenty more times. Its only an hour to make twenty, five minute tries.
An hour session after a few rapid sessions described above seems like eternity. You will see how much more accurate you've become.
Do a case study of a masterpiece. Everything about it. Copy it perfectly. Dumb it down to its basic elements. Write about it. Research and learn the history about it. Why it was made, who for and what techniques...Draw arrows and make notes about where the viewers eye goes first second and third, label what type of lighting scenario it is. Anything you can think of.
One of the things I recommend if you are starting over is art history. See where modern illustration gets its roots from the golden age of illustration, and investigate the development of 'sequential' storytelling over time in media like movies, comics, and games. That is for illustration.
For design look to things like cars, jets, helicopters, motorcycles. Bajillions of dollars have been invested in the development of such things. Read about them and see why they look the way they do compared to a hundred years ago. Look at animal anatomy once you've learned human anatomy. What type of vehicle do you see if you combine a bear and a helicopter? How about a salmon and a motorcycle?
There is a ton to learn just do it one thing at a time. But online courses will point you in new and practiced and learned directions.