There are tons of different ways to move forward but what I think the best approach for you would be is to try and focus on the clarity of shapes and form. I get that you're going for a loose approach to sketching (I do the same) but there needs to be a clarity to the shapes of value/color you apply to your image. If we take the shoulder amor stuff as an example. There's a ton of scribbled shapes there but they don't really work together in order to communicate a solid form. What you need to do is to figure out how you can arrange a few abstract shapes in order to give the impression that there is form there with light hitting it.
Clarity and simplicity is your best friend. Once the big shapes are in place, you can start finding smaller shapes to work with and then you can continue to find shapes that define form until you have a finished, well rendered painting.
You might want to try and go for a less loose approach to painting/drawing. It's something that is very difficult to do effectively and when starting out, it can set you back by making a painting/drawing difficult to work with. Try to aim for simplicity and clarity and you'll hopefully notice your work improving.
I made a super quick paintover to show how you can simplify and arrange shapes in order to arrive at a more effective image.