Forgetfulness. Think how forgetful people get after having lived a normal lifespan, now go for a few thousand+ years and you’ve probably forgotten whole centuries of your life. This is actually the premise of a solo journaling game Thousand Year Old Vampire, you have to cross out and forget memories as you progress through the game, just forgetting whole parts of your life.
There's a Doctor Who episode with that idea in it too, the Doctor saves a girl in Viking times but brings her back forever, and when he meets her in mediaeval times she has a whole library of books that are just her memories that she's written down over the years.