How do I go back to the fundamentals?
For context: I've been drawing since I was around 5/6 years old but I've decided that I would start to take art more seriously instead of just trying to draw from my mind (I'm 18 now) but I'm stuck because I have all of these ideas but I have no clue where to actually start when it comes to learning.
I've seen a lot of people say that when you practice something the wrong way for so long it gets engraved into your head, so I guess my question is how do I go back to the fundamentals?
And yes I've tried the art books, references and YouTube tutorials but I feel as though I missing something. I've spent weeks working on breaking things down into shapes but I feel as though I'm not making any improvements.
Color and paint is something I'd like to explore too, and yes once again I have tried tutorials but I feel like I'm missing something there as well.
Any tips? You tubers? Study techniques?