How can I make my still life shadow realistic?

Try looking at a real life model. The lighting will change the shadow drastically, so make sure you have set, indoor lighting that won't change in the time that you draw. Look at the model carefully. Use a smudge pen (which looks like a tightly rolled piece of paper) or your finger to lightly smudge the shading to make it more realistic. Above all else, practice your shading on easy things, such as round objects, until you feel comfortable to move on. It takes time, but in the end, you'll have something you are proud of, and that's worth it.