First up - welcome to TalkBass!

To your question, everything you’re experiencing is perfectly normal whether you’re starting out at 15 or 65. It takes time and it’s easy to feel overwhelmed. So try to view what you’re learning as chunks. Posture, fingering technique, muting, stretching while avoiding injury are all important. But they’re also very difficult to think about simultaneously when you’re just starting out.
So as you’re practicing, step through the things you’re working on in a round robin fashion if that’s helpful for you. Play a passage concentrating on your fingering. Then, once you’re fairly satisfied with that repeat the passage but shift more of your attention towards muting, etc. Then rinse and repeat. As you master things you find easiest, you’ll soon find the things you find more challenging will start falling into place.
Look at it this way - you love music and you’ve already had a great jam session. You’re already playing
music - not just your bass. That’s at least 75% of the equation right there. The rest is just technical details that will come in time with practice as long as your heart is in the right place.
As far as feeling there’s so much you don’t know, all I can say is congratulations. Realizing how much there is to know is the badge of a real musician. I’ll celebrate my 50th year of playing bass next year. And I’m at the point now where I think of myself as an actual beginner. Everything that went before was getting the details out of the way so I could actually begin to learn what this thing called “bass” really is.
It’s like studying a martial art. When you first get into it you think black belt is the end point. So you diligently plug away through the various belt colors until after some years you are awarded your own black belt. At which point you now know enough to suddenly realize that far from it being the finish line, it’s actually the
true beginning of your studies. You’ve gotten all the details taken care of so they’re no longer a distraction. And with all that now being second nature, you’re finally ready to really start learning what the martial art you’re studying all about.
So hang in there. If you keep at it it’ll come together.
In the meantime, welcome to the fold. You’re among friends and allies here. Glad you chose to join us.