Residual Parental Guilt
You do your best to raise them and then send them out into the world, knowing that you have done all that you possibly can to prepare them.
It doesn't end there. In the increasingly likely scenario that your beloved offspring do not flee the nest, one is left with the uneasiness of reclaiming a life whilst the focus of it for so long is still present and demanding attention. At least if the offspring have left home, there is just the ongoing nagging worry about their well-being. When the offshoots do not shoot off, a whole new territory of relaxation anxiety is open for exploration.