A/N I wrote this very quickly after watching TATM. It may be horrible. If it is, I apologize.
It struck River harder than her faced showed.
Back on the TARDIS, it was all cheer and small smiles and hints of 'you'll see me again,' because she knew that he couldn't have dealt with knowing how upset she really was. Right then, the Doctor had needed someone to be...not cheery, then he would have a) felt that she was being disrespectful or more likely b) realized that it was an act and try to comfort her. That wouldn't be good for either of them. The worst possible thing that they could have done was try to talk it out. No, the Doctor had needed her to be...unshakable. To assure him that Amy and Rory were fine, happy even, and to tell him what to do. To make sure he didn't sit and stew too long, that he would go find a new friend, and then to just leave.
Leaving him was the hardest thing she had did that evening, because she wanted to stay so very much. She knew that neither of them would be alright, not without each other and probably not if they were together either. But she was tired of being the Doctor's mother. Having to think 'I can't say that in front of him right now,' or 'This is what the Doctor needs from me right now.' Her parents were gone and she wanted to be held and comforted by her husband and to openly comfort him, to stay so that neither of them would have to be alone then. But she couldn't, for the Doctor's own good.
River sank to her knees. Unwanted tears pricked at the corners of her eyes as it began to rain. River hadn't realized she was crying at all, but once she noticed that she was, she couldn't stop. She felt somewhat disgusted. River hated crying, and here she was, sitting in a gloomy graveyard in the rain, bawling like a baby.
Why was she so upset anyway? Amy and Rory were alive, they were together, and they were even happy. Happy together, forever. Why couldn't she just accept that? Her parents would have each other for the rest of their lives.
And anyway, even if they were her parents, she was never their daughter. They could never accept her as that tiny little baby stolen from them at Demon's Run any more than they could have accepted Mels as that girl. She knew that Amy and Rory both respected her, liked her, and even loved her, but it wasn't ever the sort of relationship a parent had with their child. No, no, she was wrong. She was their daughter, she just wasn't their little girl, and River, wonderful as she was, wasn't ever going to give them back their Melody Pond.
River stood up and walked a few paces over to her parent's gravestone. She bent down slightly to trace the letters slowly with her index finger.
River was not alright, she was upset and lonely and she wanted her mum and dad. She wanted her husband. She wanted for something, just this once, to work out in her favor.
She wasn't alright. She should have been.
But she wasn't.