It isn’t fair, but when has the universe ever been fair? It’s big and complex and full of as many horrors as there are wonders, but fair has never been an adjective the Doctor would use to describe it. Even Time Lords are too insignificant to rate any special treatment from the immensity that is the universe, so what chance does an accidental human Time Lord metacrisis have?
The only thing the Doctor can do for Donna now is wipe her mind clean of all her memories of their travels, everything she’s seen and done.
It isn’t fair.