help me find this manga please

i tried getting chatgpt’s help but it didn’t work, i’m trying to find the name of this one manga i read ages ago. i believe the story begins with the protagonist (a high schooler) who lost either all his memories or part of his memories after an incident. and there’s this one character in this manga who i believe literally has the face of a cartoonish sun or moon, and this character is referred to as “memory” by the protagonist. also this character is only seen by the protagonist and is most probably an illusion/ hallucination. i think there’s also time travelling in this manga or maybe the protagonist being immortal or the fact that he lived for a long time with different identities in different periods of time. also another detail: the manga isn’t very long i believe (should be around 80 chapters?) and i remember the ending being kinda open and very rushed due to the pressure on the author to finish the manga quickly which could explain why it’s finished but feels “unfinished” (hope that made sense ) also i remember the artsyle being kinda flashy and very unique and pretty