Protagonist slowly loses their mind/gets more miserable, but no one around them realises why

Something akin to the vibes of We Have Always Lived In The Castle by Shirley Jackson, or Everybody In This Room Will Someday Be Dead by Emily Austin. The protagonist bottles all their thoughts in and has a completely different view of events than other people. Even if other characters in the book know something is up with the protagonist, they don't realise the extent to their madness/struggles