When you sit down to draw from your imagination, how do you go about that?
I find it's extremely hard to just sit and wait for inspiration to hit me, I don't think it really works that way for most artists and it's an unreliable method of creating. You might want to consider finding a theme and maybe creating a moodboard for it. (That means collecting a bunch of pictures related to your theme and creating a collage that shows these things together on a single board/picture.)
You can use this moodboard to create a new drawing that is based on what you chose. For example, you can collect images of certain clothes and bodytypes and then draw various models that fit these themes, but instead of recreating a reference you're creating your own.
I think if you do this regularly, you can train your brain to create new concepts from source materials in your environment (or even relying on your memory) and then 'create from scratch'.
It's probably best to let go of the idea that all original work has to come into existence from literally nothing. Art exists because we see the world in a certain way and then create based on that. Nobody ever creates something from absolutely nothing, it's always traced back from a source. Unlearning this is probably the hardest part for me and many others.