Heyho, man!
I am not that much of a pro myself, but I've been practising portrait painting for some months earlier this year. What helped me a lot was a lot of repetition, measuring out proportions using reference and studying the loomis head as well as additional info from other sources.
The thing that jumps to my mind first are correct proportions. Thats probably what will improve your work the most at this stage.
I can recommend the youtube channel from Angel Ganev. He has a lot of good videos on portrait painting and provides you with good exercises.
One of the exercises I profited the most of was selecting a portrait reference picture, putting it on one side, duplicating it and then:
1) do a drawover of the construction lines of the head over the ref
2) try measuring out the reference and reproduce it on your own
3) repeat this for at least 30-40 heads and really take your time doing this: the first few heads might take you 45-60mins each, but its worth it, man!
Check out Angel's channel here, just bear with his special style of presenting the info : )
Hope thats useful for you!
If you need some more help, feel free to contact me!
best,
Mau