Hi! I've been doing a lot of portraits recently and here's just a few things that I've done to improve:
1) Learn the facial planes! These are incredibly important... make sure to check out the Asaro head! If you get a really good grasp of these then you can stylize them to your liking. https://www.artstation.com/artwork/GX3Ax1 Do studies of them in different lighting, different angles, etc. They're not fun but they really help.
2) Value control! Zin Lim has an incredibly good channel which discusses the 5 value system (midtone, shadow, light, halftone, highlights) for painting heads as well as just general sketching. I'd suggest following his videos down to each stroke if you want to get a really good sense of it. Also remember how light works, ambient occlusion vs. shadow, etc. (check out Marco Bucci's youtube channel for some really good videos. Also would highly recommend the book "Color and Light". It's an essential for all artists.)
3) Skulls: learn them front to back! One mistake I see a lot are eyes that aren't recessed into the face. Remember that eyes aren't just tattooed on the face -- there's a socket, an eyeball, eyelids, etc. If you understand the skull the rest comes easy.
4) Proportions.... this is pretty easy. Biggest thing is just to remember to keep eyes one eye-width apart. If you understand the skull and the planes this should take no time at all
5) Face studies!! Don't really do this until you understand planes, lighting, skull etc. or they won't be productive. Once you do like 100+ they'll start becoming easier and easier. Sounds really daunting but it's worth
Faces are really tricky because we humans can recognize them in a heartbeat, so we can also instantly notice what's wrong in a drawing. I know I kind of spewed a lot of stuff here but hopefully at least some of this helps. Studies can be annoying because they may not be what you want to draw, but they'll make every drawing you do afterwards better. 
Keep it up!
