The First Person in History to Lose His Memory

My name is Chen Wuji. It is said... That I am the first living being in this world. I was there when Pangu opened the sky; I was there during the prehistoric years; and throughout the ages, I have always stood at the top. The only problem is: every ten years, I forget all about it. I have a dream every ten years, and when I wake up, I am a new person. I have repeatedly been a stranger in the world I created. When I woke up in this life, a girl named "Xiao Jiu" hid in my yard, followed by a group of people who wanted to silence her. They don't know, I just forgot, not useless. And I just want to know: Who left this jade pendant with "Don't Forget" engraved on my chest to me?