Nice work, you really did great with the shading. One can see you put a lot of effort into it. However two suggestions for improvement:
the nose under the mask: it looks to a bit to thin and a bit like cut out with a knife. I think it also would have to stick out of the face a little more. Try not using such a hard line, but those shading techniques you are well capable of.
the mouth: I think the mouth should be visible a little more under the mask.