The Doctor sometimes cried when he thought of his own personal paradox.
He cried now because he hadn't cried when he buried his own body in the sand.
He cried now because he hadn't cried when he trapped Gallifrey in a Time Lock; when the mass of screaming, writhing, burning bodies called for help from a Doctor who had turned against them in their last moments.
He knew what would have happened if nothing had happened to Gallifrey—if the Last Great Time War had never happened (not in the way it had never happened now, but if the Time Lock and the Daleks in general had never happened), or, before that, if the Second War in Heaven had never happened either, or if the Faction Paradox never existed in the first place (one could only hope, after all).
The Doctor 'remembered' everything in the would-never-be timeline, up to his last regeneration and death—though since both wars, he hadn't even regenerated as the same faces as he would have, as the general Timestream of things had split so much. He 'remembered' when he visited Mictlan, the area in conceptual space where the Celestis lived. He remembered when he promised them his body.
They didn't exist any more, the event didn't exist any more, and his remains would remain not auctioned off to anyone, wherever they may lay when he finally bit it.
Of course, Celestis was an idea in and of itself; the Celestial Intervention Agency elevated to consciousness and ideadom. The Time Lords had tried to copy them, though they would have killed off everything else in the multiverse had they succeeded. (This brought up the fact that Celestis did everything better. No, Celestis did not do everything better; only things relating to pure ideas.)
The Doctor destroyed them as he would have done to the Celestial Intervention Agency had he had the guts to, at the time when he confronted them for the lives of all those innocent people, before everything was gone twice over.
Nothing held them together now, not that time had fractured so much history wasn't even legible.
The Doctor would sometimes remember for a moment about his very brief time as Lord President of Gallifrey, when he held the Great Sash of Rassilon and slipped on the Hand of Omega and felt the Matrix and all the dead Time Lords pressing themselves onto him and trying to reach out and touch him and nostalgia would fill him, so that it was bursting out of his seams and he couldn't bear the survivor's guilt.
The Doctor would take the TARDIS to the edge of conceptual space and open the doors and sit on the very edge of her with both doors open, dangling his legs so they were almost touching the 'sky' of Mictlan.
He would 'watch' the ideas and their reanimated servants in their own little world, like one-way glass, and considered throwing off a shoe to see where it landed and see if the one shoe would break the fragile barrier.
He would get a very dangerous idea then, and hope would blossom in his chest.
It wasn't because he remembered his never-to-come deal with his body in a casket and his own death and inevitable resurrection at their hands, knowing it would never come to be, as it said on the tin.
It wasn't because they were all gone—not just his own people, but the Daleks and the Faction Paradox and the Enemy and the Great Vampires and anyone who had tried to annihilate Gallifrey. Everyone had failed, though in the end, it was exactly what it said on the tin as well—everyone had failed. Though there might have been some cause for hope because the dangerous enemies were gone as well.
It was because the Doctor could see a fault line there, over Mictlan. It ran across conceptual space like an arrow, spearing the hearts of Gallifrey and Skaro, wherever they were (lost to the rest of time). The fault line had the Matrix woven in it, the Time Lords looking up at him and, once again, pressing their 'faces' against the 'glass' and smudging it, trying to catch his eye, reaching their hands out to see if they could touch him—but they never could. They were not real, they were trapped and fated to never exist.
He was happy because there was a way to rescue Gallifrey from the Time Lock. There always had been. He could rip Celestis inside out. He could (theoretically) save everyone. But the Doctor just dangled his legs. Rip open the fault line, the Matrix comes flowing out. The Enemy destroys half the world after annihilating Gallifrey, while the Time Lords annihilate everyone by elevating themselves. No one won, except the last race to survive.
The Doctor just dangled his legs off the TARDIS, and slipped his shoe back on. It was another never-to-be moment in a world of never-to-be moments existing only inside the conceptual space of his ideas.