Need a good reason for character in time-loop to wipe her own memory.

A witch, stuck in a time loop trying to save her town and family from a Cthulhu like entity, since everyone is driven mad and kill each other at the end. Therefore she uses magic to go back in time trying to stop it.

The story starts at the beginning, with the witch being frantic, and looking for her book so she can stop the entity before he becomes too powerful. Half-way trough the story she needs to wipe her own memory, using alchemy to do so. So that she can go the trough the last part of the story, and go back in time again, effectively restarting the loop. Any good ideas for why? Or any other good ideas for how the loop will loop?