"Do you want to die?" The Doctor had once asked him that question long ago. At the time, the answer had been no. He didn't want to die. He had just found the Doctor again and here he was at the end of the universe with all these people trying to live. He would have nightmares in the years to come about being stuck at the end of the universe, screaming into the darkness alone, but right then he wanted to live.
He had never asked for the curse of immortality. Never asked to outlive his lovers, his friends and everyone else that he ever knew. To continue on when all he wanted to do was stop. He had no choice in the matter though. That choice was taken from him that faithful day aboard the Game Station. When he had sacrificed his life in order to give the Doctor a chance at saving everyone. Rose, with the help of the TARDIS, had forced him to come back. Forced him to live and so he had.
Unlike the Doctor who ran, Jack stayed as long as he could. Holding his lovers for as long as he could, all the while knowing that one day they would take their last breaths. Some of them, like Ianto, had gone to early. Others he had held until they were old and gray in their beds while he remained young. He was vain about the occasional gray hair that appeared on his own head but what was one gray hair to the fact that the same baby he held in his arms at birth, he had to bury at the end of their life.
Eventually, he gave up. He drank trying to forget. He tried to forget the pain, the lost children and the Doctor that never returned for him. He lost several decades to the alcohol as he lived in a constant alcoholic bliss. He lost several more when he met up with the headless monks. They beheaded him and stuck his head down into a monastery where he would remain alive and alone for many years. Until one day, an archeology team found him.
River Song knew the Doctor as well and they shared stories of their time together. He missed the Doctor still, he would always miss him, but he refused River's offer to see him. He couldn't let the Doctor see him as just a head. He had to find a way to get his body back. He never did though.
As the years passed he adapted into his new way of life. Learning how to take care of himself. His head grew and some days the fact that he had a big head amused him more than others. He tried not to look back. Tried not to live in the past, but if there was one thing he would say never faded, it was his memories. At night when he closed his eyes, he could run with the Doctor across a new planet. He could laugh with his child as he pushed them on the swing or he could just sit holding Ianto one more time.
Then beyond all hope, the Doctor was there again. He got to see his first Doctor save everyone aboard the space station as he took Rose on her first trip into the stars. Then only a few years later he would get to see the Doctor he spent a year on the Valiant with at the hospital on New Earth. He thought he was finally ready to die, but seeing the man he had never stopped loving again gave him the will to carry on. That will would sustain him as the drug killed the people of New Earth and he had to keep the ones on the Motorway alive.
The Doctor came to him that one last time. Though he wanted to tell the Doctor who he was, his time was finally done. The glass broke on the jar keeping him safe and he laid on the ground breathing air for the first time in decades. When Martha came he knew it was time to let go. His nightingale and his Doctor would be the last ones he saw and that was alright.
"You are not alone," He warned the Doctor as well as let him know that he would always be there for him. Closing his eyes, he finally felt the peace of death that had avoided him for so long. The sound of the TARDIS filled the air and he opened his eyes. He was once more standing by the hub in Cardiff. The door to the TARDIS opened and he ran towards it. Inside his Doctor was waiting for him.