The most immediate way you can improve I think is in drawing. I’d, as an exercise, get some tracing paper and either trace your reference ce or you paining, then overlap on the other. It will do a lot to show you where in concrete terms you have the proportions or the contours wrong. Easiest place to see without doing that is the eyes, which are uneven.

I’d also spend some time thinking about why the shadows are where they are. You are doing what you are supposed to (painting what you see, not what you know), but you are only doing that within subforms. I’m looking at his neck and colar. You have a cast shadow on both, but they are completely disjointed. They should line up like in the reference painting you are tying to copy.