(CONTEXT: You and Tom always were enemies but when you found out about his plans to kill Harry and to make the magic world his, you ran to Dumbledore to tell him but Tom found out and teleported you to the Chamber of secrets and while you two were fighting he pushes you too hard in a wall and you pretend that you lost your memory)

You are pretending to lose your enemy so your enemy(Tom) wouldn't kill you

Tom lying to you and telling you he is your fiancé