Yeah that would be because you actually don't understand something here. Your proportions are really wonky, for one. Your forms are okay I suppose but you're having trouble with either understanding how values work or understanding how to render things, but I can't tell which because this is such a rough block in.
I'd suggest doing a study, as detailed and accurate as you can muster (and it can take months if it needs to but it would help you). Get an academic plaster cast for drawing, or do some barque plates. And then draw it.
Make the proportions right, refine your line art until it's crisp and clean, block in your major shadows, and then go to town on your blending and rendering like in pic related. Go as slowly as you need to get it right but the entire point of this is to get a picture done that is so close to reality (with the exception of any bounced light in shadows that confuse the forms) that people look at it and go "wow is that a photo?"