I do agree that your reference photo is not the best. No problem - you can "extrapolate" some stuff.
Very nice light on the shoulders. Yes, be a little bolder with the dark values. A suggestion would be to make some areas in the hair darker. Look at the reference photo to see where I mean. Perhaps you could define the jawline a tiny bit more? I would say the same for the nostrils as well.
And most importantly is how you
want your portrait to be. As of now, it has a bit of a muffled look.