Sayaka is blaming herself and being blamed by some readers, but things changed exactly because Nanami initiated an attempt to have an extremely limited relationship with Yuu, falling in love with Yuu in the process. Had Sayaka or Yuu approached Nanami offering love before this attempt, then Nanami would have rejected either. And it would have been terrible for Sayaka, actually loving Nanami, to have proposed a limited relationship, especially had she done so without expectation that she might bring Nanami around in the process. And there was no reasonable basis for Sayaka to have such an expectation.
Sayaka was simply dealt a very bad hand.