Grief is usually imagined as something huge and dramatic, but here the speaker makes it small and ordinary. Their great grief is compared to "an insufferably simple supper," a meal so plain it's almost irritating. The word "insufferably" is interesting: the speaker seems annoyed that sorrow should feel so basic, so lacking in grandeur, when it ought to feel enormous.
The second comparison, "a rough kingdom," pulls the other way. A kingdom suggests a whole territory the speaker now lives in, and "rough" makes it harsh and unfinished. So grief is both a humble meal and a country with its own bleak rules.
The last lines, "It will not come / to much," are blunt and a little deflating. They could mean the grief will lead nowhere, or that it won't amount to anything grand. The mood is quiet, dry and weary. Nothing is shouted. The speaker simply sits with a sorrow that is heavy yet strangely plain, and says so with very few words.