I think the idea is that you find other people and things that make you happy. When you're enjoying them it makes the old pain feel better.

[That's how it worked for Fai when he came to Vatheon, at any rate. Memories of the tower were replaced with the memories he made here and the people who cared about him. Even though he still had nightmares from time to time he knew he could wake up and find comfort in the world and people around him.]