She doesn't feel that way for no reason, as she says in this chapter she always wanted to be able to use magic and also to escape the responsibility of becoming queen. So when she swaps bodies and her wishes are granted, she is ecstatic. But then she hears about the damage that act caused and sees the destroyed forest for herself. Knowing she has the ability to address it, she decides to rectify what she can.
TL;DR: she would feel bad just purely profiting off an event that hurt so many living things, so she wants to help things heal. It's not a self-sacrifice thing, she's not trying to give up her life or magic or anything, she wants to feel she deserves to have the magic.
I did state that it differs from person to person, Rafalga was saying it in a more general way, and sure she can try to fix the problem, feel bad all she want, I just want to convey that she has no reason to
take the blame at anytime they bring it up. She can be a goody two-shoe whenever she like but she was educated as a princess, act like one when it count, have dignity, don't get startle just from a loud squirrel, don't just look down and say "Please forgive me", look at their face and apologize if you think it's your responsibility, don't give promise, act, the big squirrel is right for every sense of the word, it just folded cuz of the little ones.
You might think I'm asking too much but good god, everyone live and walk on the same dirt, she has to be brave and got enough resolve if she want to take responsibility for another person, if she's like that then even a child could put their foot on her and she would accept it without complain when the cat wasn't there.
Like shit, the author should show us, what the villagers think, what happened around her that she wasn't aware, why do we have to get the cat to tell us every single thing while drip feeding us with little to no info on what's going on, it's just bad story telling if this keep up. Granted I'm not expecting much, but I see no point of not criticizing where it need to be.
Edit: Adding on what's I just said, what she's been trying to do is not enough to justify her wanting to take responsibility to make her feel like she deserve the magic personally speaking. Just saying, six chapters in and I feel like we still only know about 10% of what actually happening around her, took her over a week long to even realize that the fauna and flora are still withering and suffering around her area, also the only reason she's trying to reach the town was because her supplies were going down, so that can be implied that if the house has its own autonomy on getting supplies then she might have not even gone down town to check what's going on for who know how long, she is taking it a bit too lightly.
But hey, that's just my opinion, if people don't agree, it's totally fine.