Someone keeps leaving, and each time the speaker is left with a gap that nothing can fill. That's the whole poem, and its plainness is what makes it hurt. There's no elaborate metaphor, just a hole, described as "so so big," where the doubled "so" sounds like a person who can't find a better word because the feeling is bigger than language.
The speaker seems to feel two things at once: loss and a kind of helplessness. "Everytime" suggests this has happened more than once, so the pain isn't a single goodbye but a pattern, and the speaker keeps being hurt by it. The line "youre not replaceable" works as both a compliment and a trap. The person matters enormously, but that also means no one else can ease the ache.
The final line, "i just have to live in emptiness," sounds tired and resigned. The lowercase, unpunctuated style makes the mood feel raw, like a private thought typed out quickly and honestly.