8:08 AM, Sunday August 23rd 2020
Because the lines extended towards the left and down have very little foreshortening but the lines extending to the right converge very quickly. There is no way there could be such a difference in foreshortening, especially given the angle of the box. Make sure the convergences are always similar.
More than a box, it's like you drew a tapered box (which is not actually a box), like the ones we do in the rotated perspective exercise in lesson 1.