Hey! Glad you've found the right place in the forum <3
Faces are hard, there's no questioning it. Our brains have millions of years of evolutionary experience reading them for expressions, anomalies, or even telling two people apart at a glance. It's normal that we are absolute experts at finding something wrong in a drawn or painted face.
To be honest, I find you've done a good job on the ones you've posted here. To improve, keep doing more faces! You'll improve faster if you stick very close to reference pictures, no need to do them from imagination at all. Build a construction matching the photo ref like you were taught in the class, then for the rendering take your time and compare to your reference often.
It's a good idea to use references with very strong light and shadow (like your last model pic) to get a better understanding of the planes of the face, and general rendering of faces.
Good luck, and don't forget to keep it light and fun