The Doctor heaved a sigh as the door of the TARDIS creaked noisily behind him. It was the goodbyes he hated the most, parting ways with those who he had grown so close to. It was almost impossible to not be filled with regret and a lingering desire for the adventures to continue, to extend their grand travels. To never have to say goodbye.
A shudder passed through him and he groaned aloud. The process underway was, though useful, not one without it's drawbacks. The one at the forefront of his thoughts right now was the pain he was feeling. He hurt in places only regeneration could bring him pain. The feeling was one that you had to experience to describe, the best he had come close to once was when he described it to one of his companions. "It's a bit like swallowing dry toast without chewing first." This, he had thought then and still believed now to be a fairly accurate description of the sensation. It was like that, but in all the veins and passageways in his body. The raw, powerful energy coursing through every route, canal, nook, and/or cranny in his body. It's golden glow a painful visual reminder of it's flow. He had almost grown used to it. Almost. You never could quite get used to the sensation if acquiring a new, well, everything. New face, new skin, new hair, new feet, and, ugh, new teeth. He knew it never helped to anticipate the exact moment when the transformation would climax, but couldn't help but physically and mentally tense up in preparation for the new persona.
Another pulse. Strange. It didn't usually come in waves that harsh, at least not until the end, when the regeneration piqued. That wave, as would be expected, hurt a lot. "Oww!" He shouted aloud, voicing his discomfort at the anomaly. This was not okay, something was wrong, and that distressed him. Was he dying? As in, DYING dying? No. Not right now. He wasn't ready, there was still so much to do. Planets to save, galaxies to liberate. Things he still wished he could have said and done. He ran his fingers through his hair frantically at the thought of these few moments being his final ones. He was scared, though, strangely, he didn't mind. He was almost at peace, almost... happy.
Yet another burst of energy, this time causing him to actually heave his body, though not at his own will. Oh wow! That hurt a lot more than the previous. It felt like he was exploding, only a little bit at a time. The pain was enormous. Almost enough that he couldn't even express it vocally. Almost. "BLOODY HELL!!!" How very British. But it was the only thing he could think to say. It was like an overload of the senses so immense, that only the simplest curses could describe it. The feeling that there was something under his skin gained a new relevance, as it seemed to be popping out of him rather than crawling just below the surface.
A flash of golden energy, enough to blind most people, throwing him forward onto the TARDIS console. He could feel it. The overload of energy as his cells all tried to replace, replicate, and regenerate. As another small explosion burst from with in, he could swear he saw a connection of the energy between the console and himself. A shift of his eyebrow was all he could manage before the next onslaught brought any and all speculations within his mind to a screeching halt. This time, there was no mistaking it, there was definitely a transfer of the energy in his body to the console of the TARDIS. Not desiring to hurt her, he attempt to pull himself away, only for yet another shock-wave to pulse through him and consequently through the TARDIS. This time, he could see sparks dancing and skirting about the various interfaces on the panelings. The TARDIS began to shake violently, the raw power within him visible in just about every inch of the TARDIS. Little bits of it zipping to and fro at random intervals at random spots. The TARDIS shook and protested like it never had before. Something clearly disturbing the natural flight patterns and navigational objectives.
As the TARDIS began to erupt into a violent fit of shaking and noise, so too did The Doctor erupt with energy, feeling his body shift and alter in ways not experienced to him ever before, the process so painful that he could not willingly move a single muscle in his body. Not even to open his mouth to scream. He could feel organs shift dramatically, appendages adapt and rearrange, his neck growing... wait, growing? Yes, definitely growing. He was still conscious, though through every second prayed to every God he'd ever heard of to please either kill him, or at the very least make him unconscious so he did not have to bear the excruciating pain of this unnatural shift in his shape.
He could feel every agonizing development, though he could not even begin to imagine what he must look like. His fingers were absorbed into his palms, they themselves twisting and shrinking back into his arms, which in turn receded into his shoulders. His feet doing likewise, knees twisting and rotating into incredibly uncomfortable angles. His spine going perpendicular to his neck and the rest of his torso following suit. His ears seemed to be stretched and pulled till they were atop his head, his head itself growing in strange spots (such as something strange which he could not describe happening with his mouth), it's overall mass increasing, his eyes were hurting from the inside as they too grew with the rest of his head.
When all was said and done, the transformation complete and the TARDIS at rest, only then was he granted the sweet relief which he determined must be death.