I'm probably making a simple mistake with this project, but so far I can't figure out how to do what I want to do. See the attached STL. I started with a 3D scan of a person's head. I then used Meshmixer to cut out the precise section of the face I wanted for printing (the attached STL). Apparently the way I did this wasn't proper because it's causing me all sorts of problems trying to prep it for printing. Meshmixer and Netfabb tell me I have holes in the mesh, but when I try to auto fix them major parts of the model disappear. My intent is to print this exactly like a thin mask. I don't want to fill in the back part of the mask. I'm guessing there must be a simple way to do what I want. I thought some of the experts here might be able to save me hours of troubleshooting by offering some tips. Thanks!

mask.stl