I think the problem here is it seems you’re trying to learn how to shade from your mind. Something that I’ve come to realise over time is that we learn art through observation, even our most creative ideas are influenced by something we’ve seen or someone else’s art.
So to start, get a lamp, and a random object in the house and draw it out with the light at different angles using only a monochrome palette (or just a pencil and paper). Pick something simple like a book or a mug and leave out any unnecessary details like patterns or signs of wear, only draw where there is light and darkness. It doesn’t have to look good, or be a finished piece of work or anything. The goal is to get yourself to start noticing where light does and doesn’t touch an object, so that eventually it’ll come intuitively when you’re drawing from your mind. Set a timer for a minute or two per object so you don’t get stuck on one thing. If you can’t work it out for a particular angle or shape, it’s no big deal, just move onto another object.
Another thing that may help is to just copy other art and characters to work out how their shadows work. Something like Pokémon is really good for this because there are so many to practise with and some can have have fairly complex shapes while using very simple shading techniques. With this method you need be careful you’re not just copying for the sake of copying. Study the way the shadows work and then try to draw them in without looking at the reference.
In general, this advice applies when you come across any challenging art concept. When you get stuck always try to find existing images that represent the thing you’re trying to recreate, or even better study objects in person.
Hope this helps and good luck! (: