Welcome to deviant art, and digital art!

I don't have a ton of advice to offer on your painting specifically since I don't generally paint portraits, but my main suggestion would be to do a ton of photo-studies with a strong light-source (good shadows) until you really have a strong grasp on the structure of the face. A single mistake with proportions or shadows can make the entire thing look wrong.



There's also lots of youtube tutorials about drawing "the planes of the face" that I think could be helpful in drawing portraits.

Another thing you could do is to find artists who's work you like and pay close attention to how they draw (some of them might even have process videos or tutorials)



Probably wasn't super helpful, but good luck on your journey to improving your art!