Time. Time never stopped moving forward regardless of how tightly he tried to hang on to every second he had with the ones he loved. He cried at the birth of his children and then again as he stood over their graves years later. He watched buildings rise and then crumble. He walked through civilizations that rose and fell in what seemed to be a blink of his eyes. Never to be remembered by anyone but himself but even his memory of what use to be was fading.
He could still remember the way the Welshman looked all those years ago. He had promised him that he would never forget him and he hadn't, not even after a billion years. It broke his heart to know, though, that he could no longer remember the way the man's voice had sounded or whether or not the picture he held onto so tightly in his memory was the way the young man had smiled. He wondered sometimes when he had forgotten the way he smelled after crying himself to sleep holding his coat his arms all those nights. Worse, he couldn't remember when he forget the man's name or if he had even existed at all besides in his memories.
There was one man he did remember despite his years, however. The man who had changed his life forever causing him to become the immortal he was. The man who he loved despite how many times his heart was broken, or how many times the man's face changed. He was the one who tended to the Doctor in his final days, promising to bury him among the stars that he had always loved so much. He had held the Doctor's hand in his as the Doctor promised that this wasn't the end. That they would see each other again near the end of Jack's own life. Then and only then, he was to deliver a message.
The message was simple and one that Jack held onto repeating it over and over again in his mind until the words had lost their meaning. The Doctor was wrong, though, he was very much alone. Even if he was constantly surrounded by others all of his reasons for living were gone. Even his memories were failing him despite his desperate grasp on them. Then suddenly, one wonderful day, the Doctor was there in his leather coat. Jack rejoiced as he prepared to give the Doctor the message but it was too soon. The Doctor didn't know him yet, he had just lost his own people and was trying to show his young companion what it was like to lose her own planet. So instead of telling the Doctor everything, he had watched on from the shadows of his own glass jar.
Time continued forward. He felt himself giving up, ready for death to take him to his final resting place. There would be no one to burn what was left of his body or to scatter his ashes among the stars. That was alright, though, as he would once more be with the ones he loved in the everlasting darkness. Weak and tired, he sent the Doctor a message, determined to finally speak the words that he had promised to all those years ago. His Doctor came to his side, once more not knowing who he was. He was happy with the young woman again as he saved the people who were grown in the hospital. Even if Jack wanted to, he couldn't tell him, not yet. Now wasn't the day for his death as the Doctor had proven that life would go on despite the pain. So once more he left the Doctor promising him he would see him one last time.
He found his purpose in life as bliss killed everyone in New New York and he saved the people on the highway. He drained his energy daily as he kept them safe, circling endlessly in the smog filled tunnels. People lived and died on that endless highway, yet they still sang every night, their song feeling the air and giving him the strength to keep going even as he felt himself slipping away. Once more he was forgetting as his world became the endless days in the citadel with only the Matron by his side. He feared that he would meet his end before the Doctor was able to find him one last time, but just as he was about to give up hope, the Doctor was there once more.
The glass in his jar broke, allowing him to breathe the air once more. He felt weak, his heart breaking even now as the Doctor didn't know who he was. He wanted to tell the Doctor who he was and what they would one day mean to each other. He wanted the Doctor to hold him as he felt himself slipping away as his head laid on the cool cement ground of the citadel. He couldn't, though. The only thing he could do was to give the Doctor the message as he had promised to do all those years ago. Using his own voice for the first time in as long as he could remember he spoke the Doctor's words.
"You are not alone."
Taking one last breath he felt the darkness overtaking his mind as he finally was allowed to join the ones that he had lost so many years ago. The sound of the TARDIS returning filled the air and he stood up on his own legs as he hurried towards it, ready for his next adventure with the Doctor by his side.