This is meant as an addition to Joachims answer, so I won't repeat what he already pointed out.
I'm sorry if this sounds nitpicky, but it seems to me like you start the process of drawing with the contour of the person, then you add the outlines of the strongest shadows to give the flat contour some dimension and lastly you fill the outline with different shades of grey to mimic the contrast you see in the photo. That approach is what leads you to get the shadows wrong.
Personally I find it easier to think of my final picture in 3 main tones that I apply in precisely this order:
- The base color (a medium tone)
- Shadows (a darker tone)
- Highlights (the lightest tone)
Admittedly I usually paint with acryllics, so setting highlights is as easy as adding a brighter paint. In drawings you either have to leave the highlighted areas bare or erase your lines in the area.
In your painting I see only 2 of those tones: shadows and highlights.
To show you what I mean with 3 tones I opened the original photo in MS Paint and converted it to a 256 color bitmap. This reduction of colors makes the areas of the body more visible. The shadows are converted to a blueish purple, the highlights almost a pink, the medium tone a brown. To make it more visible I highlighted the medium tone in a second example.
It covers the sternum, her right shoulder and most of her right breast and the right side of her left breast. In your drawing these areas are so light that you cannot effectively highlight the parts of the breast that are actually hit by the light.
The gap between her breasts, right above the bra, is almost white in your drawing, which suggests it stands out enough to be hit with lots of light. In the photo it actually has a medium dark tone with gradual transitions towards even darker shadows left and right.
The center of the models left arm in your drawing is almost as bright as her neck. But since it is even further away from the light source than the neck, it actually has to be much darker.
In my experience, these errors happen when you concentrate on small areas of the thing you draw instead of the whole thing. You replicate the contrast between colors in very small areas (like right between her breasts) without keeping the overall tone in mind.
Just as Joachim wrote,
Treat the entire body as the one single entity it is.
Start with the contour of the object or person, then start lightly shading all the areas that are not directly hit by light, but ignore the cast shadows in this step. Gradually add more strokes to these areas to work out the 3-dimensional shape. This creates your medium tone. Only then concentrate on details like cast shadows. Since you put the medium tone there before, the overall impression will be much more cohesive.
Additional note: Her bra in the original photo is black. In my experience pure black and pure white are extremely problematic colors in painting and drawing. If you actually fill such a big area in flat black, you'll probably smudge it around, creating a mess, and all dimension is lost due to the flat color.
Instead of replicating the photo, I would treat the black bra as dark grey (as you correctly did) and add shadows and highlights according to how the light would be reflected off any other color. Meaning, I would add shadows to the underside of her breasts and keep the triangular tops and straps of her bra lighter than they appear in the photo, just to emphasize the 3-dimensional shape.