Ok great to see where you are at. When you are first getting started it is very important to really focus in on the mapping out of your shapes as accurately as you can possibly get them. If you put a shape in the wrong place and commit you end up having the other shapes off and require fixing, which increases painting time. By taking just a few extra minutes early on to measure out your shapes, to compare your shapes, and be sure they are placed and drawn accurately will make the rest of the painting process, working out your values and edges, much much easier.
You should flip the images horizontally and vertically so that you see the shapes with fresh eyes. This should be part of the process and if you are already doing that, keep doing it more. The professional artists will often flip images or use a mirror to see with fresh eyes as many as three or four times a minute as they are working when things really get flowing. You can also back away...actually get up and back away...and doing this works for shapes as well as checking values and edges.
Also, using a soft edged airbrush at low opacity will be good to brush values down in a way that builds up forms and value gradients that suggest volume. Also for adjusting edges. start with your focus on shapes though as you are very close now, and we just need to push a little bit closer and you will be good to go. These are pretty good for your first start at this...so you are on the right track.
Keep up the good work.