Great question. There are a few dozen ways to approach this issue, so let me come at it from a slightly different angle than the above post. When we speak of "mental states" collectively in this way, we have to look at what conditions or events could affect women in such a massive general way, or literature in such a profound one.
The war broke people mentally, and often permanently. Women who had lost loved ones, friends, or had witnessed the carnage as nurses or residents, or had lost their own homes and possessions to the war, or even suffered in the privations that followed the conflict took these tragedies into their mental states for the rest of their lives, and this is reflected in the literature of women in the second half of the 19th century. The hardships themselves became art, stories, and protest, sometimes all at once.
This is a fascinating topic. I am not certain that you will be able to receive a definitive and singular answer. I would say that the mental states of women in the writing of female authors played a vital role in their works because it was the one realm that could not be penetrated by male patriarchy. Puns not withstanding, women in post Civil War America were limited in what social, political, and economic domains into which they could tread. Any advancement was not socially acceptable nor traditionally sanctioned. Women were met with male resistance each step of the way. The retreat inward into the psyche of the woman as well as the emotional state of female characters and protagonists was the one domain where women were free to be who they are and who they wished to be. Rhymes not withstanding, it was in this realm where women could be independent and envision an existence that was transformative of social, economic, and political reality. On another level, perhaps the desire to depict the emotional states of women could have been an attempt to humanize them and make them more of a figure to be appreciated by male resistance. In trying to illuminate the humanity of women, perhaps some authors sought to make the issue of women's rights one of human rights. When Woolf conceives of a "Room of Her Own," she is making a clear statement that there has to be some domain where women are free of external subjugation and repression. Perhaps, mental states were this one area.