Hi Meihum! Wow, your art shows a great deal of understanding when it comes to contrast and specific details like hair. This is definitely attributed to your experience with detailed animal drawing! Really cool stuff.
You seem to be really interested in drawing portraits, which also tend to be pretty detailed. Some things that could be helpful for you may be doing studies of the musculature of the face and the skull underneath, and studying how it can move for different expressions. While doing this, it may be useful to consider the three-dimensionality of these muscles. Your own face is a very valuable resource for this! (You could also ask a friend to model for you). After you check out the muscles of the face, Try drawing different expressions using your own face as a reference and check out how they move to create different expressions and planes.
some other excellent resources that may help you in your journey to portrait drawing are the loomis head and the asaro head . After getting used to these simple guidelines you may find that your understanding of the forms of the face and head improve by quite a lot, and your understanding of detail and contrast will really come through.