I think its the difference in impressions, The bully from middle school literally betrayed our protag and he only remembers the bad, meanwhile Yan girl picked him up from suicidal depression even if she also later went psycho; however her picking up from his lowest is something he can't forget even when her obvious mental ills revealed themselves.
Even though she betrayed him to, Yan Girl was completely sincere in her initial actions, meanwhile all the positive middle school bully had was implied to be mostly false (At least what i can glean)