Isnt the answer obvious? If he wants the time loop to stop, stop killing her!!!

He didn't know that killing her caused the time loop to occur, he thought that she was causing it before she died. It was only when she died in front of him when she took the arrow for him that he realized that her death was the cause. That's why in the current loop, he prevented her death from the arrow.
The mix-up is easy to make though, I only know for sure rn bc I just reread the story lol