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The Doctor had changed in the centuries that he had been alive. Even though he still depended on Jack to be beside him at all times, he was no longer the timid man with the broken vocabulary that Jack had once feared he would have to take to flatholm Island in order to be cared for. Instead, he had relearned to be himself with his mind almost completely healed. Therefore, as she had regenerated in the sunshine, Jack was hopeful for the next body that would run with him as the Doctor. The last thing he expected was to see was the Time Lord regenerate into his tenth body. He watched on in concern as the Doctor, paused in his self-examination of his new body to realize that he knew the teeth in his mouth. He held out his hands, staring at the familiar hands that Jack had once held. Jack still knew every inch of that body as he had helped clean it, dress it, and care for it. He had also spent hours just holding that body against him calming the Doctor from another panic attack that had racked the body frequently.
"What," the Doctor asked one last time before the familiar look of panic overtook his features. "I can't, I can't, I can't," he repeated as he fell to his knees reaching up to grab his hair in his fist.
"Sweetheart, listen to me," Jack told him gently snapping out of his own shock. He fell to his knees in front of the Doctor, gently taking his wrists in his hands trying to pull them away from his hair.
"I can't do it again," The Doctor whimpered, his voice cracking.
"I know and I won't make you, but think about it, Doc. This isn't the same as before. Back then your mind and your body were separate but now you are able to think clearly. You also don't have the scars that decorated that body. This body is new and back to the one that we knew before everything that happened," Jack explained to him calmly. The Doctor finally let Jack pull his wrist down and he looked at Jack with terror in his eyes.
"But this wasn't supposed to happen. I wasn't supposed to be in this body again, it was supposed to be something new. I can't look at this body every day knowing what happened to it and how I used to be in it," the Doctor told him close to panic.
"I understand but just like last time, you won't have to go through it alone. I will be here with you every step of the way. Now what I need you to do is think. Is it possible the Master messed with your regeneration due to all the havoc he caused with your body recently," Jack asked him calmly.
"It's possible but it should have been impossible for me to change back into this body," the Doctor explained again repeating himself in his panic. Jack took him in his arms holding him tight. The Doctor clung to him, twisting the familiar fingers into the fabric of his shirt.
"Don't worry, Sweetheart, we'll figure it out together."