You can't draw your own stuff because you don't practice drawing your own stuff. You don't practice drawing your own stuff because it's hard. It's hard because you don't practice drawing your own stuff. It's a common thing to see, because compulsive photostudiers tend to get very competent at doing really nice photostudies, and then when they draw from imagination it looks AWFUL by comparison. It's incredibly discouraging and tends to put people off imagination drawing. I had a long phase where I was terrified of drawing from imagination for basically that reason, it devastated my self esteem to think I was good and then draw such beginner-tier shit.
Your imagination shit will almost always look a bit worse than your studies, because using references is the equivalent of taking an open-book test, you can just look up the material whenever you're stumped. You might get a better 'grade' at the end (a nicer looking piece) but how much of that was from what you know, as opposed to what you could look up and parrot? Which isn't to say 'never use refs', especially if you've got a project where the outcome is more important than learning, but you need to actually test your imagination drawing to get better at it. It's a very different skillset to observation drawing, and while observation drawing might 'build your visual library' as people like to say, it won't actually help you use it, because it's like your dumping your 'books' into big piles and not organizing or understanding them. Drawing from memory, reducing things to basic concepts (like simplifying into basic shapes) etc basically consolidate and catalogue your mental library.