you've got a great amount of studies, that's very good!
I think you're focusing too much on outlines tho, it's like you're trying to copy what you see to paper by just putting an outline around it. that makes your drawings very flat, and mistakes show up easily because you're not sure which lines are important and which aren't.
it's a lot more important to understand what you're drawing, in terms of 3d shapes and weight, force, etc. those are the things you should be trying to describe with lines. have a look at other people's drawings and see how they seem to wrap their lines around 3d shapes, and how they vary line weight and strength to add a little bit of extra zing