In Catching fire, when Katniss chooses to attempt to save Peeta in the arena, do you think she realizes that she is choosing to leave everything she has ever loved and worked for behind? Or is this just the ultimate way to repay her dept to Peeta?

Both, probably. As far as she was concerned, everything she loved had already been taken away from her the moment they announced the rules of Quarter Quell. She was convinced that there was no way she'd survive it because Snow was after her. But Peeta had a chance. It was the smartest option, no matter what she was leaving behind.

she said it was to repay the debt because Peeta was protecting her the Hunger Games.

Describe the conditions of district 12

Honestly I think it was both. She loved him very much( or she wouldn't do it) and she wants to repay him. For helping loving and caring for her :) <3333

She realized there was no possible way she could win, so decided to save Peeta.