What is meant by, "Themselves compelled at midnight and at noon to watch with steadfast eye their destinies"?
Let's look first at the overall theme of James Garber's poem. He is a man ruminating over how brief and unsatisfying life has proven to be. This poem may be the most existential in the series, in that it seems to address the brevity and absurdity of existence.
Love cannot save him or anyone else, for, eventually, love is lost. The "faces of friends and kindred / Become as faded photographs, pitifully silent," meaning they have died. Those who are gone cannot help us.
The response, when one is left alone, is to "reproach mankind," or to be angry at the world. This, too, ceases because, according to Garber, all of mankind (those who are referenced by "themselves") must also recognize their unfortunate fate, which will be to lose loved ones. The phrase, "they are compelled at midnight and at noon" suggests that time will not release them. They must "watch with steadfast eye their destinies" implies that they must be strong—must carry on—in spite of their pain and losses.
By the end of the poem, there is no optimism, but there is a collective sense of existential dissatisfaction. Garber tells us, from the grave, that none of us can do anything to relieve the soul's longing. We will always want and will inevitably end up dissatisfied, but must carry on nonetheless.