The language of Gallifrey is exact. Its definitions have a terrible sharpness.
There are at least five words for forever and one of them means a thing that exists (existed) as Gallifrey exists (used to), outside of time and space, eternal, a constant. Always-is-always-was-always-will-be.
There is no past tense for this word in the language of Gallifrey. Either forever or never. No longer forever means never.
Sometimes, the Doctor thinks, the humans with their vague words and single continuities have got it right. If there is a single continuity, a linear thread of history running from was to is, that thread is the Doctor himself, and so Gallifrey was. He knows at least five reasons this is not technically true in the complicated physical languages of the universe.
He and the fine silver thread of his lifetimes still like to believe it.
The room was well-lit when someone said forever to him, forever in the sense of Gallifrey, but it flickers dark in the palace of his memory. Flickers dark the young boy's skin and single pulse, like his then, first-life, so young, so new. Shadowed. Scintillant.
And Koschei speaks honesty, from where he knows. In the friction of their bodies, in the bright searing moment of shared knowledge, he weeps the truth into Theta's mind.
Forever. That truth which spins on its axis outside of time. I'll be with you forever.
Death still comes to forever things. Deaths, one by one, change what is and what was.
The Doctor knows it, looking on the boy's pleading eyes later, thirteen lifetimes later for his lover. He knows it, flinching from the skin of the man's stolen body.
The mind inside still wants him, still warms to him. He dreads the touch and the heat of closeness as the Master leans over him, deliberate, affectionate. Together they fight entropy, and it seems a terrible irony. The love between them still exists, but the Doctor cannot reach it. The murder that took this body creates an impassible barrier.
He knows of death then, but doesn't learn it well enough.
The word Rose means when she says never going to leave you is not a Gallifreyan word at all. It's a human word; a declaration of will.
He treasures it. It makes him think of other things he's treasured: supernovae, ice cream cakes, operas and friends. They all belong somewhere in his sequence, his record.
But there's only the record now, spinning away til it hits the end and spits clicks of static for no one to hear.
There is no word for forever in the new and ancient language of the Doctor, species of one. None for never either, and no one to tell him that lacking these concepts in language can drive a man insane.