Mid-Death Crisis

“Mid-Death Crisis” (so christened by Joanna Harris) was the name for the tendency of immortals to grow weary of life after a few centuries, feeling as though they had seen it all — a type of supernatural depression. (PROSE: Vampire Science)

History
Many human vampires were hit with Mid-Death Crisis around their four-hundredth birthday. Joanna Harris, over the course of her unlife, had seen vampires walking into the sunlight to die simply because it was all they could think to do, and others starving to death within their coffins for lack of willpower to get up at night and feed. Harris herself had been hit rather hard by Mid-Death Crisis but pulled through through focusing on her survival instinct. (PROSE: Vampire Science)

When she told the Eighth Doctor (another near-immortal, and about her age) about her concept of Mid-Death Crisis, he was struck by the idea, acknowledging that he had sometimes grown forlorn of the constant sense of déja-vu that came with having visited so many places as he, but also arguing that in his case, whenever he thought the universe was getting too dull, something new always came up. (PROSE: Vampire Science) Indeed, before (TV: Pyramids of Mars) and after (TV: The Rings of Akhaten) this conversation, the Doctor occasionally displayed some bitterness about his unnaturally long life, and he was always overjoyed to discover "new things" (TV: The Snowmen) and "impossible things" which even his prior experience couldn't explain. (TV: Last of the Time Lords, The Snowmen, The Bells of Saint John)

After centuries of life on Earth as an immortal with a mortal memory span, Ashildr became immensely jaded with the smallness of human life, having become all she could become in a pre-space-exploration world. She grew increasingly desperate to find a way to leave the planet to find something new to do, even falling in with alien criminal Leandro. The Twelfth Doctor discussed the downsides of immortality with her, noting that he shared her world-weariness and, like her, envied "young" individuals such as Sam Swift, who were, unthinkingly, "so alive, so happy to be alive" in a way he and she could no longer ever be. (TV: The Woman Who Lived) Much like the Doctor or Harris, Ashildr seems to have pulled through this rocky patch in her eternity, and no longer seemed quite so desperate and restless when the Doctor met her again billions of years later in her subjective timeline. (TV: Hell Bent)