When in doubt, use reference! A flashlight and a mirror is all you need. If you don't have breasts yourself, that part might be a little tougher, but you can extrapolate from what you do see. For when using yourself isn't an option, you should be building up a reference image folder of people and objects in various lighting situations. There are also 3D "life drawing" programs and sites out there that let you play with lighting.
Here are a couple of edits on the shadows. The left one has more of a frontal component than the right one.
The thin neck threw me off when I was shading it, so it's a mess. Necks have quite a bit of surface detail that make them great for showing the lighting direction, but this character has such a thin one that it's difficult to fit anything into it.
Edit: I didn't touch the hair at all, but remember to shade the hair in accordance with the light source too!