Through pain she was able to grow and learn more about herself.
I don't know if we can draw any conclusions about Natsume's own feelings on the matter. But she obviously loves Shiori a ton in her own way. I don't think she's the more mature one necessarily in this story--I get the sense she also can take her own share of this pain and learn to grow too.
Maybe a good idea for a sequel about her own experience here, if Amazaki-sensei wished too. I'm plenty happy with this as it is either way.