It is indeed an awkward position, the form not sharply articulated in the original. The dark body shadow and the very dark background in the original permit the edges of that form to be "lost," but you have very substantially lightened those darks and, so, placed upon yourself the difficult task of trying to paint something other than as it appears in the original.
Still, this hand will be a form more suggested by value shapes than by detailed drawing. You should try to accurately copy the value shapes -- not "pretty close" or loosely, but in the same contours, shapes, and value as you see in the original (which is admittedly hard to see in such a small format). Forget that you're painting a "hand." Get the value shapes accurately depicted, and a hand will appear.
Again, don't get "pretty close." Depict
this subject as she is, not a generic one. For example, in another area you have let the shadow shape on the neck angle down pretty much straight from the center of the chin, with a sharp edge, all the way to the sternum. In fact, in the original, the contour barely drops below the chin, if at all, before moving markedly to our left, under her jaw line, before descending in a relatively softer edge. That distinction makes all the difference in the world in depicting the form. It is the only reason to be painting light at all.
Such attention needs to also be brought to bear on the hand. You don't need to be thinking, "Now I will place the knuckle." You need to be thinking, "This is exactly the way the light plays out over this part of the form." Get that right, and the knuckle will "miraculously" place itself right where it belongs.
Be careful of the length of the fingers that you can't see very well in the original, and be mindful of their usual tapering in thickness as you extend them.
This seems an apt opportunity to refer you to another recent post on careful observation (
Click here.)