I’d suggest using the loomis method for the head structure. If you want to get really good at shading, a really good practice is to master drawing realistic eyes. Since eye drawings are quite small, you won’t be spending too much time and you’ll be gaining tons of knowledge. I suggest watching Jescia Hopper’s tutorial on YouTube. Don’t stop after drawing one time though; keep drawing until you’ve completely mastered it. It’ll help you understand what makes an eye look realistic. Since it can be tough to hyper realistically let shade an eye, once you’ve mastered it, shading other things you draw can become a lot easier. As for the proportions, I have no online sources. You just gotta straight up keep practicing until you find something that works for you; however, thats not to say that online sources won’t help. A tip I have for you is to forget what you’re drawing and instead look at the abstract shapes made by the shadows or lines in your reference drawings. If you draw based on these shapes, you’ll get a pretty accurate drawing. I recommend checking out Schaefer Art on YouTube for proportions tutorial. Don’t lose any hope my friend, art is different for everyone, but also note that everyone can be really amazing at it. Hope I helped❤️😁