Rose! You fibber you! You CAN draw a straight line! You just proved it.

Drawing from the elbow or shoulder really does make a difference, using just you wrist gives tight lines and restricts your ability to create good lines - its works well with circles too.
The first cube looks great, the shading could be a little smoother, but you've got the right idea there.
The hatching looks fine too. I'd just suggest that you go a little darker in your tones. Change the direction of your lines and get them closer together. They can virtually touch and blend together on their own to achieve the shades that you want.
Rose, it does take practice to get the results you want ad it won't happen overnight, but it will happen.
1. You can draw a straight line. Your hatching shows that and you know if you draw from the elbow or shoulder that it helps. Try a whole page of nothing but straight lines. Its a fabulous exercise to loosen up with. Go one direction, then turn your page and draw in the opposite direction. It may seem silly to draw just lines, but it does help.
2. Uniform pressure takes practice too. Try creating a value scale. Make 10 one inch (or 1.5 cm) squares on your paper. Using just one pencil, create 10 different values, leaving the first square white then progressing the tones up the scale til you get the darkest shade that you can. It will take a few layers of graphite and some time and patience, but you will see what you can achieve with just pressure.
3. Edges don't exist. Only tones do. Look carefully at something in front of you. Do you see a sharp edge? No you don't. What you see is something that blends into the next shape or tone. This is what you want to achieve in your drawing too. Don't think of edges, think of light and shade. The edge of something is created by its position next to something of a different tone, not by a distinct line.