Great studies, now moar updates! About the self-doubt, those feelings are really familiar to any artist. I have lots and lots of them, seeing so many people progressing so much faster than me and at a very young age too. Right now I have about 1-3hours to drawing daily (little more in weekends), I have dayjob and small kid. So often I get the feeling I'm not going to make it and I'm studying it wrong. Also not having art friends in rl is sometimes very frustrating as it often feels like even my closest friends don't get this and only thinks this is a hobby for me.
But then I try to remember that I'm not in any hurry and we all will get there at our own pace. The master artists are the same people as us, they have struggled and struggle the same things than we. And they don't have any magic hands but the very same hands that we have and our hands are capable of doing the very same things as theirs. And you love doing art right? The feeling what you get when you are drawing and painting. Try to hold on to that! Don't compare your journey to others, your journey is yours and unique, nobody else has the excact same. Some people learn faster than others but that doesn't mean they are the only ones who will get there and be better artists. You do this cause you love it and you wan't to get better at what you love and you will when you practise it. Just enjoy the ride as well. And you are already doing very good and improving! Just keep at it :)
Here's also Jonathan Hardesty's interview if you haven't seen it. It's very inspiring and he's talking about these same things. https://www.youtube.com/watch?v=3Hnyjx2eBg8