Biggest thing sticks out to me is you face feels a lot older, I think mostly because jawline is less defined and features are less sharp. Your face while believable doesn't feel as young, athletic, and idealistically stylized as the reference. basically the reference character is more sexy and yours is more ordinary.
compared to the reference your fabric feels too thick. I think it needs harder folds especially at the armpit area. It seems to be poofed out like if you are wearing many layers of winter clothing. But in the reference it looks more like heavy cotton that is oversized.
the microsurface of the cloth also seems a bit off to me. Hard to say precisely what causes the problem but it looks a bit plasticky and too clean. Not sure we should be able to see the grain of the fabric like that either. It seems like you might have put that detail in there because it felt empty without it, but I think the emptiness comes from not having good details in the folds, and perhaps needs more variation in color as well (stains, fading, etc).
Thank you @Alex_J, insightful feedback. Head anatomy and cloth sym/sculpt were indeed the things I had less experience with, I'll study and practice more
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https://www.artstation.com/artwork/Jly8gz
Head anatomy and cloth sym/sculpt were indeed the things I had less experience with, I'll study and practice more