mostly a face edit. The features are basically in the right places. You made the shape of the face too round though. And youre showing the face too much from the front, should be turned slightly to one side. Shape of mouth is off, and you could darker the shadow areas more. looking at it now, you could also make the width of the hair on the left side smaller.
also I noticed the shape of the breast on our right is off. see here
whats the middle ground between 1 pixel outlines and no outlines? AA the outlines so they blend more with the surrounding shadow.
also, the areola isnt flat, is curving with the breast, so a little shading will help it.
you also have alot of contrast on the shading of her stomach. but in the ref the shadow is basically pretty solid, with some subtle darker spots. I tried to make the shadow more unified there in my edit.
EDIT:
I was thinking it might help you more to show you whats wrong with the mouth:
so I just traced a higher res version of your ref, then blew up your mouth and traced it and put it on top. You can see clearly the difference in shape. Now the challenge is how to translate that shape into a smaller number of pixels. Of course it wont be perfect but it can still be closer than what you have now. Hope this makes it clearer.