I really, really enjoy this and I'd like to help you out a bit! The shading around the nose and upper lips are a bit too contrasted. I suggest that you use a bit of a lighter color, mainly so it isn't so intense. This is just a suggestion to help with the depth of the image. If this bothered you, I apologize and you may delete this comment. I hope my input helps you with future projects.

Keep calm and draw on!