Hi there aspenpando!
I fully understand your struggles. My own art skills developed with drawing manga, and it wasn't until years down the line that I got stuck in areas similar to your own.
What I found is that manga makes for a very poor base to work and learn from. At its core, manga is a cartoon, a stylized representation of what we see in real life. When you use it as your guide to learn how to draw, you do not learn to understand correct anatomy and end up struggling because your brain can't 'connect the dots', so to speak. You're basically trying to fly when you haven't learned to walk yet!
The best way to remedy this is to discard manga tutorials and focus on realism - gesture studies and using real people as models to study from are the way to go. This probably sounds like something you don't want to do because I'm guessing you really enjoy the manga style, but fear not, you don't have to give up manga all the way. :) The idea behind this is to make your brain see real life and give you the ability to put it down on paper first in a way that makes you understand how things go together. When you know how to do this, you will be much more capable to apply this knowledge to your manga drawings and give them the life and dynamic feel that you're yearning for now.
Knowing how a body is put together, how the joints work in movement and what gives faces their special expression from real life studies is a base that you can stylize later. It's like putting down the foundation of a house and building the walls, then painting the walls in the color you like to make it what you want it to be. You can put motifs with wallpaper and whatever you like on there, hang up pictures, put up shelves, but you'll always have a sturdy wall making that possible.
I hope this wasn't too confusing! If you want to get started with gestures but don't know how, I recommend these articles:
https://line-of-action.com/gesture-basics-1-line-of-action/
https://line-of-action.com/gesture-basics-2-torso-and-hips/
https://line-of-action.com/gesture-basics-3-joints/
I'll do my best to answer any other questions you may have. :)