A/N - I wrote this in an attempt to make sense of some of the Doctor's unusual behaviour in canon. I'm happy enough with most of it but there are a few things that I still haven't made my mind up on fully, but hey, I finished it anyway. Eight parts, episode introspection for the most part and a few original scenes.
Part 1 - Library
He knew from the first time he met her that he was going to spend a lot of time running. That time in the library when she touched his face and looked at him like she did, expecting the same in return. She loved him, he was certain of that much, and she didn't seem like the type of person who would without encouragement. He realised that in the future he would give her reason to love him, and knowing that didn't feel fair.
Whatever happened to meeting someone and falling in love with each other in that order? Wasn't that how it was supposed to happen? But there she was, her breath mingling with his, spilling her 'spoilers' out through her eyes: familiarity and tenderness followed by gut-wrenching pain. He knew it shouldn't have, but it really pissed him off.
He wondered if he was ever going to learn, if he would ever stop letting humans fall in love with him. He thought he had learned his lesson after the last time, when it hurt him, almost as much as it had hurt Rose, to say goodbye. To admit that it wasn't going to ever work because of their species incompatibilities; he would never be human enough for her. Now he was finding out that he was going to do it again. His future-self was an absolute idiot.
She was atypical as humans went, though; she was sharp, almost as sharp as he was, and she advised him to be less emotional. Normally they wanted him to be more like them. But even so, it would always end in the same way: he wouldn't be able to provide what she needed from him and he would have to leave her. He didn't want to travel that well-trodden path again. He decided then that even though he liked her name, he was going to call her only 'Professor'; anything more familiar might encourage her.
And so the running began.
He found out quickly how to hurt her, "Who are you to me?" He could see it in her eyes; his not knowing her was breaking her. Not being allowed to provide the answer cut her all the more. Knowing that about her was dangerous information. She said these were his rules. He thought his future-self callous for providing such ammunition to a version who would use it readily.
She had his screwdriver and he didn't give his screwdriver to anyone. That was something he was sure he would never change his mind on – unless it wasn't 'given'. He supposed the fact that he entertained the notion was what made her feel the need to prove to him wrong. She apologised and then whispered his name to him – letting him know exactly who she was to him, and who he was to her.
A sickening mix of conflicting thoughts and emotions coursed through him. Some time in the future he would feel like that again. A spark of hope was followed quickly by a tightening in his stomach as he felt his choice being removed. He knew he had to trust her, but that didn't preclude him from resenting her. He didn't want his life to be mapped out; he wanted to be able to choose his path. He pushed the hope out again, it felt wrong.
He had wondered for some time if he would be ready if the opportunity presented itself, to finally put an end to the tiredness he felt and the drag of every moment. Four thousand and twenty-three people could be saved, and if he wanted to do it he would have to die. He was ready, it seemed. Donna would be fine; she didn't need him anymore. Everyone else was gone and he'd only known River a day.
He made his choice and then she robbed him of it. She wouldn't let him go. She was going to sacrifice herself, not for any of the four thousand and twenty-three, but for him, and for herself. He admired the selfish motive behind the selfless act; she reminded him of himself. He called her 'River' at last: a confession that he believed everything that she had and hadn't told him.
She sat in the chair and made sure that he knew exactly how much he would come to love her and then she died, taking his free will with her.
As he sat handcuffed, waiting for rescue, he found himself wondering what exactly he would do to make her love him with such fervour that she was willing to sacrifice herself for times he hadn't lived yet. He wondered how he would come to love her, a human, in a way he hadn't thought possible, a way he still didn't believe was possible. The enigma terrified and excited him in equal measure. He knew then that he had already started to fall.