As an exercise you can learn a lot from masters like Holbein that added an amazing sense of perspective to their portraits. Find some of their masterpieces and try to trace them applying the loomis or reilly method of construction, simplifying the features. I know it won't be as dynamic, but later you can deal with other views of the human head. Steve Huston has a course on constructing the human head, and Ramon Hurtado has a great gumroad on it as well https://gumroad.com/ramonhurtado?sort=newest#TPlxp. You can give a look at Stephen Bauman's channel, he is always doing livestreams so you can see how he does it https://www.youtube.com/channel/UCqEWLv392F0J9rN7jE1-f3w .Keep in mind, you are building a head, not outlining an image. Good luck!

Hi Thomas, thanks for sharing! One thing I like to do to get better at drawing anything is once I draw from observation or memory using my reference I trace the image on tracing paper or photoshop and overlay the tracing on my drawing. I did this with your drawings and attached it here. If you take a look you can probably see specific areas that can improve. I notice generally head size and nose placement related to the mouth. I think you captured the head angle pretty well across all drawings. Using this method really helps me get better faster because I can see clearly what I need to work on. Hope this helps keep it up! Drawing from memory is tough!

Hey Thomas, i think you should not yet focus to much on likeness. I think it would be good for you to focus on structure first. Maybee try the Loomis-method, it provides you a basis to build your features on and if you then take the right mesurments the likeness will improve to. Also your references seem to be very expressive, maybe try less expressive faces first.