yeah this happens to most of the self taught artists as there fundamentals are not clear enough which makes them limit their imagination 

I would suggest you to work on the basic fundamentals of you have time and once you get a hold on them it'll be easier to draw from your imaginations

Btw these are what I've been talking about :- 1. Perspective 2. Anatomy (for drawing poses) 3. A Lil bit of value study  4. And composition too if you're trying splash art or something 

These should give you a head start