For the feeling of inadequacy: "There's no shame in being beaten by a master."

Quite frankly, it sounds like you need a writer's workshop. Or, in this case, Roleplayer's workshop. (I'm planning on holding one of these on the Guild Discord in the near future, I don't know if any are done on the Guild itself.) But short of that, you need to find a RP mentor. Someone that will take you through their Advanced posts, so you can see what they did step-by-step, so you can understand how their posts get to be however long, as well as analyze your posts and offer critiques and suggestions. 

And if you don't read a lot, read more.