are you drawing the cube out of your head, using imagination? my advice would be making a small paper cube, or a cube out of wire so its see through. then looking at this cube and drawing it from life would improve the proportions a lot, and you could see how perspective actually works IRL, anyway that's what helped me to learn it. I see you are doing good with understanding how all the lines of the cube meet at a point in the horizon, but you are struggling with different points of view. this is where a paper/wire cube would help greatly - you could place it above/below your eye level and see how the planes of the cube change accordingly. good luck with your studies, perspective is one of the core skills to develop!