Haunted
The memory of Rose Tyler haunted the Doctor. She would burst through his mental barriers at the most inopportune moments, nothing but a memory yet so much more powerful, regardless of whether he was in the library by himself or out adventuring with his companions. The first time, he had relived their entire time together. Flashes of Charles Dickens and Slitheen and nanogenes filled his immense mind – any space not occupied by Rose Tyler was quickly filled. The Doctor could not bear to lock away the memories, and so slumped to the floor, a broken man, each time this happened.
Martha had never found him like this, though she had seen him red-eyed and tired upon occasion when he entered the galley after an episode, desperately trying for a nonchalant expression as he rummaged around for a banana. But now, Martha too was gone. Though he had enjoyed their travels together, the Doctor had been horrified to realize that he was slightly glad that she had left. Martha, though a brilliant friend, had just entered his life too soon after losing Rose.
Donna, on the other hand, walked right into an episode the day after they had visited Pompeii. Donna sat with him as he cried in silence, her head resting comfortingly on his shoulder. The Doctor, being the rude and not ginger person that he was, never acknowledged Donna's support, preferring instead, to pretend that he never curled up into the tightest ball possible and cried his eyes out; he always came bouncing back to the console room, pressing buttons and spinning dials with a flourish, promising grand adventures.
Though he never admitted it, going back to New Earth had drained him. Never before had he not appreciated a second adventure on a planet, but his first visit to New Earth had been too recent. Rose had been too recent.
When he regenerated a violent regeneration and destroyed the console, the TARDIS had repaired it herself to its former state of organic glory. Now a new man, the sight of the room still triggered painful memories of friends lost. Memories of a fiery redhead, a brilliant doctor, an immortal ex-conman, and… the Doctor, even while he was irrevocably changing, could not bear to remember the loss that had plagued him for the years past, the loss that he himself had ensured was now permanent.
As he regenerated, his mind clung to a memory now two lifetimes ago. Exchanging physical pain for mental, the Doctor re-lived the first time he had kissed Rose Tyler. The first time he had kissed her, he had sent her away.1 A brief kiss on her forehead about as far from intimate as possible. A kiss that could have led to something more, had he not regenerated, had he not lost her, had he not sent her away with the Metacrisis. He had seen the timelines and had he not been the coward he was, she would be with him still. Of course, it was more complex than that, but really, that's what it boiled down to. As the fiery pain left the Doctor's body, the mental pain failed to subside and the Eleventh Doctor realized that, no matter what happened, he would always love Rose and he would forever be haunted by her loving memory.
1 The Parting of the Ways, Minute 14:07