Calmseer was indeed a spectacularly good Returned—the last of the old guard, so to speak. She Returned, in fact, in order to save the life of her daughter. She of course forgot this once she got back.
She did complain about not being able to do enough for people, though she had that personality even before Returning. She was the self-sacrificing type who took care of those around her and always had a kindly attitude. She died from an illness she caught while caring for the sick family who lived next door to her. (They'd lost their father to the same illness, and while all eventually recovered, Calmseer herself came down with their disease and passed away from it.)
She didn't give up her Breath because of what Lightsong assumes, that she felt so guilty for not being able to do more for people. Instead, she saw her daughter come through the petitions line. The woman was brought by her husband, who felt he had no other option. His wife had the same disease Calmseer had. She remembered, at that moment, why she had come back—indeed, she remembered her entire life (that's common for Returned the moment before they give up their Breath)—and gave away her life to heal her daughter.