How we cite our quotes:
I killed you, I think as I pass a pile [of rotting and burned bodies]. And you. And you.
Because I did. It was my arrow, aimed at the chink in the force field surrounding the arena, that brought on this firestorm of retribution. That sent the whole country of Panem into chaos. (1.11-12)
Katniss refers here to an event in her past (it takes place in Catching Fire, the second book in the series), when she made a choice in the arena to protect herself – without knowing the effects it would have on her home district. Because of her, indirectly, all these people are now dead.
Caesar leans in to him a little. "I think it was clear to all of us what your plan was. To sacrifice yourself in the arena so that Katniss Everdeen and your child could survive."
"That was it. Clear and simple." (2.23-24)
Here, Peeta continues to send a message that he's an innocent figure who would "sacrifice" himself to save Katniss's life. And, whatever else might be true or disguised in his speech with Caesar – whatever else might be propaganda that he has to say – it's pretty obvious that he would sacrifice himself for Katniss, just as she would do the same for him.
It's impossible to be the Mockingjay. Impossible to complete even this one sentence. Because now I know that everything I say will be directly taken out on Peeta. Result in his torture. But not his death, no, nothing so merciful as that. Snow will ensure that his life is much worse than death. (11.62)
Katniss feels like she can't go on, not because of what it means she has to give up, but because of what Peeta would have to sacrifice. It would be easier to undergo punishment and torture if it were inflicted on just her directly. But for that torture to be inflicted on someone else is more than she can bear, and we get that. It becomes a kind of mental torture, where all she can do is imagine the terrible things that are happening to Peeta. She doesn't want to sacrifice him.