i think there is a perspective problem: on the reference, the distance between the tip of the nose and the border of the face on the right is much smaller than on your portrait. The nose is also covering part of an eye on the reference, and this is missing in your painting. Have you tried flipping the canvas horizontally? This helps spotting that kind of problems.

Fist of all, you got the color down really well (I always struggle with that). And the linework is good. One thing that helps me is double checking that individual features are in proper relation to each other and work outward from there. It seems like the eyes could be more horizontally alligned and the right side of the head is closer to the eye. Maybe check the angle of the jaw too? I've found quick sketching the same head multiple times close together helps you see what parts of your drawing look correct and which should be corrected. Each sketch will be different and you can combine the good stuff into the final drawing. Keep up the great work, though, it's looking good :)