In all honesty, they were doomed from the start.
As Sakura always had, she changed significantly when he wasn’t looking until one day he finally did. She was no longer the bubble gum, love-struck, hopeless genin or the fiery, off-the-handle, repressed teenager, but a woman. A still easily irate woman, but grown just the same. She worked too hard and trusted too easily, even after everything she had been through. Sakura shined, and when she looked his way he thought he would go blind.
Their relationship went slow and casual. They had relearned each other in their different roles until one night the cautious flirting escalated and he didn’t have the strength to tell her ‘no’.
He was happy, and she made him actually feel good about being happy. She taught him to remember without remorse and to honor those that have died by living a life they would be proud of. He did- with her, in her, for her- but, as with anything else in his life, perhaps he was too late.
If he thought he had felt old before, as their relationship continued the age difference became a strain. Her vibrancy that once drew him wore him out. His calm manner that she once found comforting now bored her. They ran out of things to talk about and his older allure faded as she matured and figured out just how full of shit he actually was. She slipped from his fingers and he could not find a way to hold on. Kakashi failed her. Not because he couldn’t protect her, but because he couldn’t be anyone other than himself.
After a particularly long mission he came home was not completely surprised to find it empty. Usually, when Kakashi lost someone, he could visit their headstone and continue one-sided conversations.
This was so much worse.
He had never completely said good-bye before, choosing instead to hang on to the guilt of the loss, and with Sakura it would be no different… except for the fact that he never had anything to say
Kakashi carried many ghosts with him throughout his life, but never before was one of them still alive.