what kind of brush are you using? it looks like you're drawing with a very soft brush. if you choose a brush with a higher hardness, then the edges should be more sharper when you draw or paint.
here's an example. i use clip studio paint, but all good flexible drawing programs should have a hardness option for their brushes. (sorry for rough examples i did them quickly lol)
View attachment 654
i'd recommend changing up the hardness of your brush as you draw or paint... some things you might want a softer look, like shading from a soft light source, or you might want to define shapes or edges, so you'd want a harder brush for that.
as for editing your drawing to make it look sharper... i think you'll need to manually lasso the shape around the hair, then cut the soft edges to define it. i don't think there's a filter to make it look as exact as you want it.
here's what i mean, using my own example:
View attachment 655
erm..... hope this helps!
Screenshot from 2019-03-03 19-59-43.png
