Natsume's Book of Friends (2008)

S6E1 The Days Eater

Natsume rescues a youkai from a pot, and in return, he is transformed back into his younger self. However, since he is already quite young, this change only leaves him disoriented and muddles his memories. As he grapples with uncertainty about who he can rely on, he must find the inner strength to identify his true friends amid the confusion.