To remember one man, one thousand years. There is more pain in that one thought that can't explain without realizing the loss of his humanity. (Because humanity is brief, and all about endings)
He still feels, yes, feels happiness, love, anger, and despair. But, no, it wasn't about remembering a man for a thousand years because for the almost two thousand he was in the ground he'd remembered, only slightly worse for the wear. No, it was living and feeling for a thousand years, it was travelling and laughing and fighting and fucking for a thousand years and only seeing one face every time he closed his eyes.
Sometimes he thinks that Ianto must have hated him. To remember one man for a thousand years is to understand that god can't exist because no god could be this cruel.
He goes through stages of being There are centuries that he spends disconnected from everything, drinking and fucking his way into oblivion, charming and cunning.
Then there are others in which he is the captain again. And he saves the world, again and again, and is almost always dying. He tells a hundred men that he loves them and a hundred more women as well.
To want one man for a thousand years is to know that every moment he continues to exist he is moving further and further away from when he was last with him. Every moment is a moment he regrets because there is no end to this. No time that he is finally able to let life go. He doesn't compare all the others to Ianto, never sees the point because they aren't him.
A millennia passes, and he is back on earth, lying in the dirt where the Thames House stood. There is no relief, there is no absolution. Only the pain of loss.
So he takes retcon, enough to kill a hundred men. Enough to forget a million years. And he sleeps...
When he wakes, he is crying and the name on his lips is Ianto Jones and he knows he is lost.