If you're working with something with perfect symmetry, make sure you understand where the light source is coming from in terms of creating depth. The light source from how I see it, can come from either above, below or directly in front.

If it was me, I'd first work out the planes and how they would work in a 3D space. Right now I think you've gone into this with very little thought of how you're going to give this depth.

I might make an edit. It would also be helpful to supply the pallet you're using, so that we can help you easier. BTW, your english is just fine.