I also have this problem. But I've found its gotten easier as I've learned to break more complex things into simple shapes like cubes and spheres. For example, say I want to draw a car. I cant really imagine the whole car in my head. But I can imagine a box, and I can rotate the box in my head. So if I decide on a general perspective for the box and put that down on the paper first it gives me a good starting point for the rest of the car. Its easier for me to imagine the car inside the box than it is to just conjure it straight from the blank page. References also help out. Sometimes its easier to recognize a desired aesthetic or design aspect in another picture than it is to make it up on your own. And so a lot of the time I kind of splice various aspects of things that give me the right vibe together to create my own design.