When she said 'forever' in that moment, standing over Petyr's dead body, she meant it. She had nothing else in her life, nowhere to go, no one to protect her, no one to love her. When she returned his kiss and whispered 'forever' to him, they both believed it.
Even when Winterfell is hers again, they believe it still. But her home was not won back by him; another man gave her that triumph. He was only one man, after all, and no army would follow him.
Forever becomes now. He feels the separation before she does. Touches become less frequent, moments of passion intense but as short as possible.
He knows the proposal is coming before she does. She's so happy. Bran is restoring their home to its former glory and the hero who won Winterfell is his right hand. And right now, while we can, becomes, we must not, someone could see.
He tells her to accept, after all she can't refuse. Bran's man won them Winterfell, gave the throne to her brother and is promising a world that will always be out of his reach. She cries but nods - forever was never theirs to share.
He packs his meager belongings the day before she's to be wed and sees her for the last time.
He asks how long she'll love him for. Between sobs and sniffles she whispers 'forever'. He savors the lie, no longer interested in the cold hard truth of the world, knowing that when she holds his babe to her chest and watches her family grow he will fade from her mind. And when he is dead and gone and not even a mention of him still exists he will be erased from her thoughts absolutely.
He dies gradually, grateful for the lingering moments to dwell on and dream of what their forever could have been.
