May the odds be ever in your favor.
Hmm. I think, for me anyways, it was the scene in Catching Fire where Peeta’s heart stops. Her reaction to him almost dying was heartbreaking and very telling (I mean, even Finnick noticed).
As to when Katniss realized she loved him, I gotta say it’s probably the scene in Mockingjay where Peeta tells her ”I must have loved you a lot.” She didn’t realize how much she loved him until (at least at that point) it was too late.
IDK, this is just off the top of my head since I don’t have my books with me but yeah. Hope this helps!