You've shown some good control of value... especially in the renders of the metallic forms (helmets, etc). You need to work a bit more on measurement and proportion.

The greatest help in the world for you would be working from a simple white plaster cast (the antique...) properly lit.

Try to find a white plaster cast to draw from... think about the "notional space" that your drawing will take up (if you had to fit it in a rectangle, that's the notional space)

Measure the width of that into the height. It may be a fraction. Get a rough rectangle of that size on your paper. Now you're ready to find the proportions of the nose and eye (if you're doing the cast of a head) or the head (if you're doing a statue).

Everything is measured. Use a sighting stick, not a pencil.

And use a strip of paper to do your measurements!! This is critical, when you're starting out.

Find the height of the head from the chin to the top... vertically. Then use your strip of paper for that... and use the head measurement to locate all major locations on the figure.

After a while, you'll find that you're going to know the measurement before you get your strip of paper out. And that's when your eye will start to see things in proportion. Exciting, huh? It works, too. And a cast doesn't have to take breaks like a model.