All three made human mistakes in my opinion.
Yami is still struggling to change from her old days, still has the same self-destructive tendencies, she did it to Yuu and Hikari in each of her meeting with them and this time she dragged them both. But she also the most wounded of all.
Hikari is used to living in the warm sunlight that she isn't used to darkness (Yami's bertrayal and her perceived Yuu's bertrayal), her optimism no longer holds. She had the choice of just normally confronting Yuu about it and instead she delayed it and ended up destroying their relationship when she can't hold her feelings anymore.
Yuu is indecisive, as shown by when he still tries to care for Yami despite planning to confess to Hikari, this
broke the dam holding back Yami's feelings instead of her just closing it down for good. But that's the main theme of this story isn't it? Choosing between Light (Hikari) or Darkness (Yami).
They all made mistakes in their progress of maturing beyond teenager, that's how I take it.