ok, I've embedded the image for easier viewing which will help with us assisting you in the areas of concern to you.
first let me say what a great job you've done, all with no stencil
I can see the 'muddy' area you mention, if you sharpened the eye socket and nose cavity edges it may help. Paper shields will be of help with that (and the foggy area on top of the skull
)
the only tip for correcting mistakes is to go ahead and correct them, use it as a learning experience. usually taking it back to the base colour (bone colour in this case) then redoing the details will be the route to take.
you haven't mentioned what you are painting on or what paints you are using - both bits of useful info to assist you better.
Twood is right though, normally if you don't point things out most people won't see it, we are our own worst critics.
Because we all airbrush we may point out things you hadn't noticed or even think about and because we like to help people progress you will likely get advise wether you ask for it or not, its not meant to be critical and destroy your confidence its just us trying to help you improve