Something that once felt painless has become painful, and the speaker only understands why now. The poem is built around that delay. "It didnt hurt back then" suggests the speaker used to leave, or say goodbye, without much feeling. Now the same act carries real weight because of one new fact: this time the leaving is permanent.
The speaker's feeling is a quiet, aching regret. The poem never says what is being left behind, whether a place, a person or a life, and the poem doesn't explain what "for good" means. That vagueness makes the poem feel raw and private, as if the speaker is admitting something they can barely say aloud.
The mood is sad but restrained. The lines are short and plain, with no decoration, and the final line lands like a held breath let go. The title, "Little_Mystery," adds a note of the unexplained, leaving readers to fill in the missing story themselves.