In order to create a depth you need to broaden your value range. In your piece I am seeing maybe 3 values. If by “painting” you mean realism, then you are going to have to at least double your value range. I used about 8 in mine and also blended my transitions were called for a smooth shade. You can learn to see the subtle changes in value in your reference if you look close enough. If it is more of a stylized realism that you are going for then you could probably get away with 5 values. If you do the squint test you will see most of the subtle values changes merge and this will help you see more of the shapes that make up the light and shadows. I think if you just practice taking reference and squinting, to get the major shapes of dark and light with just one dark and one light value, it will help you start to figure out how to build what I would call an underpainting. Then after you get comfortable with this, you can start to add a middle value into it. Keep practicing this technique and you will soon be seeing more values in your references and will become comfortable knowing where to put those extra values. Hope this helps give you some items to practice. Happy drawing!