I lost this really good Loki fanfic where he finds out that he's a Frostgiant waaaay before the events of Thor go down, kid Loki finds out by touching the Casket of Eternal Winter.
Then he decides to 'protect himself' and to do that he needs to become a shadow, to be forgotten. And he does, so much so that people wonder where he has gone and his trainer is hesitent to train him extra because he will think Loki will take it as punishment.
Furthermore, Loki slips up with a fight with Thor and because of this his parents know that he knows.
I think it was on A03, so please help me out!