Bai's short poem is about missing someone so intensely that it feels physical. The speaker says that sometimes, not constantly, the longing hits hard enough that their "insides cave in," as if a building were losing its supports and falling inward. That image makes grief feel structural rather than just sad.
The last line explains why. The person who is gone was "the only thing holding me together." Without them, the speaker has nothing left to keep their shape. The poem doesn't say whether this person left, died, or simply drifted away, so we only know the loss and its effect.
The mood is quiet and raw. The plain wording and the short lines feel like a confession, said without drama. Opening with "Some times" suggests the feeling comes in waves, catching the speaker off guard. It's a small poem, but it leaves a heavy ache, and the title "twenty" may hint at the speaker's age, though that's only a guess.