Very good job. Now, this is what you can do to improve it. The thumb needs some more body (thickness), but most importantly the separation of lights and shadows is not perfect. Unfortunately, nothing less than perfect will do when it comes to realistic drawings. for example, I am seeing a lot of veins and tendons on the hand. these details are all in the shaded plane but they look as bright as anything else on the lit side of your drawing. I think that is your main issue, I would do away with details like that for now and keep things simpler. Does that make sense to you?