Nikky's poem is about the moments when feeling runs ahead of language. The speaker admits that sometimes there's only one word available, and it's a swear word. What makes the poem interesting is that the same word covers very different states. In the first line it seems like overwhelm or frustration, a kind of mental blank. In the second, it's sadness, which we might not expect from a curse. In the third, it's plain anger, the most familiar use of the word.
The repetition builds a pattern: "Sometimes" opens each line, and the lines get a little longer and more pressured before the poem ends on a single capitalized "FUCK." That last line feels like the shout the speaker has been holding back, a release rather than a conclusion. The mood is raw, blunt and honest. There's no attempt to explain or make the feelings pretty. The poem suggests that when emotions get big enough, vocabulary shrinks, and sometimes the crudest word is the truest one.