2D sprite lighting issue in 3D space

So I have sprite in 3d space and I wanna make sprite be affected by light from 2 sides (For now it's only affected from 1 side). I want to make my sprite works exactly like e.g. 3D box. Here's question: what should I change/add/remove in my sprite shader? I use default diffuse sprite shader with some minor changes.

GIF with issue