Neurons.

I read today that the human brain produces around 10 million new brain cells every single day. But for the lack of utilisation, they die sometime later. Some estimate, that we lose 70 million neurons a year, neurons that are gone, disappeared completely replaced by the new cells that are clean and fresh and contain no data, ready to be utilised, to be filled with new information, experiences, memories. But what happens to the neurons containing the memories that could never be restored once uncontrollable power of our own body decides that the cell is no longer needed (and the memories locked in it are forever gone).
There is no Undeleter programme to restore the memories from the dead neurons. Once they are gone, they are gone. With them gone are the memories that you will never grieve about. Because since there is nothing you remember, maybe there was nothing to remember, maybe there was nothing.