Gerard Manley Hopkins was a man who gave his life to God, becoming a Catholic and then a Jesuit priest. However, he struggled between his dedication to his faith and his desire to be a poet. He could not reconcile the two. Battling depression that might well be considered a serious mental illness today (e.g., bipolar disorder), Hopkins did his best to serve the church. He ended up in Ireland, feeling lonely and desolate. The poems he wrote at this time are referred to as the "terrible sonnets" because of the terrible sense of melancholy that overtook his life and was reflected in his writing.
"No worst, there is none" is a poem by Hopkins that reflects the speaker's abject misery. Hopkins wrote that there was nothing worse than what he was feeling. His grief seemed to go beyond grief, and he called for comfort and relief—we can assume the first from God, and the second from the Virgin Mary:
Comforter, where, where is your comforting?
Mary, mother of us, where is your relief?
We can also infer that his prayers are not answered. His pain "heaves" within him, for long periods of time; he suffers the sorrows of the world. For a short time, he seems to feel some relief...
...Then lull, then leave off.
However, it is short-lived. Fury is personified: it "shrieks" that no one may linger. My sense is that life demands continuing pain (so Hopkins seems to believe) until death. ("Let me be fell...")
Whatever Hopkins has felt physically or spiritually, he is then tormented psychologically:
O the mind, mind has mountains; cliffs of fall
Frightful, sheer, no-man fathomed.
The torments of the mind seem the most difficult to bear—he is surrounded by fear, the kind that would accompany one on a tall cliff, with a sheer rock face, never before navigated by man. One who has never experienced this can have no sense of how horrific the experience is, thinking it "cheap." But Hopkins knows differently:
Hold them cheap
May who ne'er hung there.
Hopkins then recognizes the short span of endurance one has to deal with such fear. One may find some comfort under a shelf in the rock ("under a comfort serves"), but everything comes to an end. Days die when one goes to sleep. And death ends all life.