Alone

"The greatest darkness anyone will ever have to face is the darkness within himself." - Unknown

He knew it was time to let go. He'd spent his time mourning, now it was time to move on. He needed to go out and meet someone new, find another adventure to go on, be the hero people needed him to be.
Heroes don't kill their friends. They give up their lives for them.
He knew it was wrong of him to think that way. He knew that it wasn't really his fault, that it was only time working in ways that he couldn't prevent. He knew that. But he had had so much time to think about his mistakes these past few...few...days? weeks? months?
He didn't remember. Time didn't pass quite the same way for him, and he never really cared to check the date. He didn't know if the time passed slower or faster than he felt it did. No, no, that's not quite right. He didn't really feel the time pass. He just knew it was passing. There was quite a difference. All he was capable of feeling was nothing but hatred and sorrow and self-loathing, along with a physical, empty-like pain in his torso that was comparable to having his organs pulled out by a beast with great claws. Sometimes, there was another pain to accompany it, one that he would inflict upon himself in fits of rage and grief. There were times he didn't remember how the bruises or scratches were made, but most times he did. He'd destroyed whole rooms on the TARDIS, breaking apart beds and smashing bookshelves and punching walls until all of his rage was released and he could only collapse onto his knees in exhaustion. There were times that he would sob for what seemed like many hours and only a few minutes at the same time, until he wept himself into unconsciousness, only to awaken in a bed in a different room. Sometimes he would lie in bed with no thought or feeling, only staring at the wall or ceiling until he either slept again or found a reason to get up. Other times he would leave the room and lock the door for a later time.
It was his fault, all his fault, and he knew it with all of his fury and grief. He couldn't stand the man that he was, nor could he escape him. He was eternally trapped in the body, mind, and memories of a man who should never have meddled in the affairs of others. Who should never have existed.
There were times when his lizard-like acquaintance (What was her name again? It doesn't matter. She'll be gone after so many years, anyway.) would contact him, try to invite him to tea or tell him about a strange happening. He would act like he was interested, or that he was busy with something else, and would come up with some clever excuse to not go and investigate. She irritated him and, on one very bitter occasion, he let her know. She didn't call for quite a while after that. He didn't realize just how much he missed her calls until he found their...conversation amongst his thoughts of hatred by surprise. He didn't want to apologize. She probably didn't want to hear an apology, anyway.
It was such a horrid life, the little hole he would constantly try to crawl out of while only succeeding in making it deeper. He didn't need anyone's help. He was a monster of a man, hurting everyone that he reached out to. It wasn't just bad luck, it was him reaching out for temporary enjoyment from people he knew he'd eventually have to lose. He'd ruined so many lives...he'd lost so much... What was the point of it? Of living? To continue going on until he died in his own misery?
Ah, yes, now he remembered. It was his punishment; his own personal Hell, just in case some of the cultures he visited were wrong about an afterlife. To live by himself with his thoughts, with only a gentle hum every now and then from the TARDIS that surged a new memory of some sort from the depths of his mind. What was this Hell for, again? He knew it was punishment, but for...for...for something bad. Something he did. Something he did to...
I am the Doctor, and I killed the Ponds.
The thought that always consumed his mind in his fits. The thought that drove him to create a new room in the TARDIS that would be dedicated to destruction; an action that had become mechanical long ago.
I killed the Ponds.
Sometimes he would only think it; other times, he would scream it until his body collapsed once more to grief and exhaustion, leaving him to only breathe it out in shaky, tear-stained gasps for air that he didn't want to breathe.
I killed the Ponds.