I have had similar problems in the past. Selecting hair can be particularly difficult in real life photos. I think in this case I would forget about trying to mask the hair, because it has blended in to the grey background. I would try the dodge tool for this one.
I had a quick stab at your photo
Levels adjustment (set white point)
Dodge Tool (targeting mainly highlights), then dodge out the grey backgeound, set a low exposure near the hair and use multiple brush strokes).
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