How I manage to do it is by changing my own perspective. Instead of thinking "I'm looking at a cube", I focus on parts of it. For instance, at the very center of the drawing, there is a small square with nothing else overlapping it. That small square has two triangles touching it's top left and bottom right corners. By trying to perceive those 3 figures as the main part of it, I was able to stop seeing any depth.

Another way is to turn the entire drawing around. The absolute top right 90° corner and the absolute bottom left 90° corners can be viewed as one aiming up and one aiming down, or as left and right. You essentially turn the drawing into a rhombus in your mind with extra details