The strange translateion of Psalm 95 in the English liturgy of the hours does us a disservice by translating the phrase, "Do not grow stubbern." How unpoetic and detached from the traditional reflection upon the "heart" is that? I don't know the Hebrew, but the Latin says, "Nolite obdurare corda vestra." I don't see why "harden not your hearts" was considered by the translators too distant from the plain understanding of the people of God. I think most people, in the west at least, have an intuitive sense what hardness of heart is and how it feels.
There is a nice, poetic book I read in my youth called Hinds Feet in High Places. It is by Hannah Hunnard. In it a deer named Much-Afraid learns to overcome her hardness of heart. The climactic scene, which I won't spoil for you, shows both why it is necessary to let your heart be vulnerable before God and why it is scary--no one likes pain and suffering.