This was written for the drabble challenge "Block" in the LiveJournal community who-contest. A bit under 450 words. Enjoy :)
He builds a wall in her mind.
He's blocking her cries as he's blocking infinities that have no business inside her head, fingers pressed anxiously tight to her temples—slipping, delving deep. He can feel it now; he can touch it, the DoctorDonna. The concept appeared unlikely before, otherworldly even. A Time Lord's brain grown out of and fused into a human's, and she was not even possessed, she just was. He couldn't see how… now he sees. His metacrisis self was a brand new being, a subtle blend, his face only with one heart and the feelings and thought processes to match. Donna is so, so human…
…but he senses the hints of Time Lady in the twirling tracks of her thoughts, wide and fast and aware. She tastes time and touches a billion possibilities, all stretching boundless into the might-will-is-has-been. Sparks and flashes from those are dancing across her synapses, singing, shifting, embracing. Time loves her and threw her across his path, over and over again, amazing loud wonderful Donna; time wraps itself all over her consciousness, and he sets to the excruciating task of prising it away.
Time is on her like fire, eating up her life's strength. The simmering lethal heat helps as it reminds him that this glory is dangerous; time will take and devour her, leaving nothing but ashes behind. He will not let that happen. To save her life, he maims her new miraculous mind, pulling away and trapping the knowledge that is too much for her to sustain.
Her memories go. He has no choice; she has seen too much and all of it interlaced, time and him everywhere, inherently part of what poisons her. The Doctor has to go if the DoctorDonna does. Carefully clinical, he does the cleaning thoroughly, cutting every hint, every detail to push back and lock into the deepest darkest corner of her psyche, too far to be uncovered again without a trigger. He shuts it all away, safely, leaving her the best protection he can manage as a farewell gift. When he pulls back, she collapses.
He catches her, pulling her to his chest, and feels the feeble thrumming of her single heart. Donna lies limp, unresisting as he hoists her up into his arms, holds her anxiously cradled for a minute. Then he lowers her so he can set the coordinates.
Time to go home, and he loses her twice. She sleeps, oblivious to the hidden layers of her own being. What he took from her weights him down like lead in each heart.
He delivers her safely, and then wanders off, alone.