I'm similar, honestly. I've spent countless hours trying to become a good drawer and a good 3D modeler, but I lack the ability to see something clearly in my mind's eye, and it feels like it's always held me back.

The thing is, I've actually worked as a 3D artist for a number of years. Some people even tell me I'm good. I suppose if that's true at all, it's because I've learned to use the computer to do the visualizing for me.

For some other artists, their process seems to be that they first picture a 'target' image in their mind, and then take steps towards that target until the target is reached. That seems impossible to me -- supernatural stuff. I almost don't believe they can really do it.

My process is closer to first finding some reference images, then taking a step in a random direction and asking whether I'm closer or further away from those references. I'm not necessarily trying to copy the references exactly, I'm just trying to match their level of quality. Then I take another random step, and check again. If you repeat this process enough times, you'll edge closer and closer to something that looks good. You'll also develop a vague sense of 'taste', around which random movements tend to produce more favourable results, and which random movements tend to produce more ugly results. It's a painful process, but it's doable.

I guess what I'm trying to say is that the ability to visualize isn't a prerequisite for 3D modeling.