If he was honest with himself it started long before the London Olympics, the feeling of dread and fear that sat in his stomach like a bolder. If he was honest with himself it had started on Krop Tor, it had started with the Beast's words. Those words had planted the seed and from there it only grew during each adventure.
He knew for certain something was coming after the London Olympics, every cell in his body could feel it. Their time was coming to an end; it was what he'd fear from the very beginning, losing Rose Tyler. A part of him wished he'd never let himself get this close to her, but the bigger part of him knew he would never trade it for anything.
Knowing the end was coming did crazy things to the Doctor. He wanted to push her away to maybe lessen the pain for the both of them, but he couldn't bring himself to do it. Instead the hugs lingered, the sharing of a bed became a nightly occurrence. He'd show up in his borrowed jimjams from last Christmas and hold her as she slept, and often he'd stay until she woke up. He took her to safer planets, beautiful planets, showing her as much as possible.
In a moment of distress he asked her how long she was going to stay with him even though he already knew her answer. "Forever" she'd said. His hearts had filled with joy for a moment before he remembered it would not be his forever. He still smiled at her, showing her as best he could how much that meant to him.
Then came Torchwood and the Battle of Canary Wharf and it turned out he'd been right because she was gone. She was safe of course, over there in Pete's World, but he would never get her back. He'd never hold her again, he'd go every day without that tongue touched smile he loved so much. That was it, he loved her and she'd never know.
When he'd finally figured out a way for them to say goodbye he still hadn't told her and perhaps that was for the best. She could move on, find love again. She could have a family and a safe life, one thing he would never be able to give her.
Of course her tears and her confession of love haunted him every day. He continued to travel, even with other companions. Martha Jones had been brilliant but he'd driven her away, his grief over losing Rose had been so strong he'd treated Martha horribly. Then there was Donna, Donna made him better even though he'd never tell her that. Donna also brought hope. There was something about this fiery woman that allowed him to hope again, hope that something wonderful was coming.
And of course, he was right once more.