How do the setting and her daily life reinforce the idea expressed in the line "I am all longing"?The Wife's Lament
The general storyline of "Wife's Lament" is that a woman has been married but, due to some trouble with his family, is now outcast. This is her lament, as the title suggests, as she is now living in "friendless exile." After being forced to leave by her husband's family, she is wandering and alone. The line to which you refer comes right in the middle of the actual setting in which she finds herself (though it's translated a bit differently in the version to which I'm referring).
They forced me to live in a forest grove,
under an oak tree in an earthen cave.
This earth-hall is old, and I ache with longing;
the dales are dark, the hills too high,
harsh hedgesoverhung with briars,
a home without joy. Here my lord's leaving
often fiercely seized me.
Her setting is stark and cold and lonely; it's not surprising that she finds herself full of nothing but longing. The next lines tell of her wandering through the forest from dawn to dark, pining for her lost love. The sterility of her environment matches the sterility (lack of love) of her heart. She is longing, and her surroundings have nothing to offer.