Putting aside the manga using the tired "crazy lesbian that obsesses over the female lead and hates the male lead" cliche, shouldn't Diane realize at this point that her "love confession" was going to be rejected? She seen in her interactions with Leo and Fooly that there's some form of romantic tension between the two of them. This confession was bound to end in rejection.
Also
@MarqFJA87,
@Tamerlane is right in saying Diane is a man hating psycho lesbian. Re-read chapters 103-110. Diane showed many signs of being that kind of character. She was always fangirling over Leo and resenting Fooly's existance. In chapter 104, Diane said that she'll take Fooly down with "no mercy" and in chapter 107, she blames Fooly for making Leo weak and that Fooly's companionship with Leo is unacceptable. This chapter just confirmed what a lot of us were already suspecting.