Jack didn't forget anything. He lost everything. It set him down a different path than the one the Moon had intended. A darker, more violent one.

Humans have a Name on their wrist. Pooka's have a Song in their hearts. Jack Frost can't read his Name; Bunnymund has only been hearing a Song for the last three hundred years. It doesn't help that they've been getting closer to each other, when both Name and Song say there's someone else out there for both of them.