Are you self-taught, attend a college of art, or had a mentor?
Personally,...I would like to find a private ( non-university based) art school or mentor that could provide formal and rigorous training in the fundamentals of 2-dimensional art.
At the moment, I am doing the Riven Phoenix program, “How to Draw the Human Figure From Your Mind”. It's been excellent. I am about 25% finished.
Check out this website. You might find quite a few resources.
I have degrees in art, but I’m self taught. They never taught much realism in college. Abstract was pushed quite a bit as well as the idea that art is anything you want it to be. I did have a few good instructors. Not going to lie, but, I’ve learned more since college than I did in college.
The website link I proved here was quite a lot of info on it. It’s also the largest online museum in the world. At least, that’s what they proclaim.