Comment by panini-chan on Luna

MC's memory loss is an integral part of the story. And yes, he lost it because someone had to stab him with the memory loss dagger thing. MC's mind was already breaking and he was just (TW) constantly killing himself due to something he learned. They had to wipe some part of his memory to save him and stop that cycle.

I'm not so sure how much you would like to be spoiled but saying who stabbed him with the dagger and the reason why he broke was basically the whole story XD