A/N: I'm using the idea that the Doctor and River's timelines are running backwards to each other here. It seems like Moffat is really playing with this, so I don't know how long this story will remain relevant but from what we know at the minute it seems about right.
No spoilers for A Good Man Goes to War here.
She'd never really understood how it must have been for him. Watching her pull further and further away from him every time they met. He'd told her of course, how it was like a kick in the stomach every time he did or said something and received that blank stare from her.
For a few shining years they had travelled together, neither of them saying it but both implicitly knowing that this was the middle, the perfect time. The day she left the Tardis, they would begin to lose each other.
So she stayed for longer than she meant to. There was always another planet to visit, another world to save, another excuse to put off the inevitable. Until eventually a call from a long forgotten friend forced her hand and she left.
Ever since that day she could feel him pulling away from her.
He didn't mean it of course. Mostly he was oblivious to her winces when he inadvertently stamped on her heart with whatever he was saying. As time went on, she got better at hiding her feelings.
But it never stopped hurting.
It hurt when she slid her hand into his pants during a particularly intense kissing session and he jumped back stuttering and blushing and waving his hands about wildly. It hurt, but she smiled through the pain.
It hurt when she whispered innuendo in his ear while he flew the Tardis and he jumped back stammering through his reproach and almost flew the Tardis into a black hole. It hurt, but she couldn't help but laugh at his discomfort.
It hurt when she kissed him and he flailed about wildly and ran away afterwards. It hurt, but the next time she saw him she was the same old flirting River.
Finally it hurt when he didn't recognise her but she had prepared herself for this day.
But when he didn't trust her? It killed her.