The poem "My Ex-Husband" by Gabriel Spera is a poem about the speaker's ex-husband. The poem is formatted as an explanation; the speaker is explaining a photograph to someone else, perhaps a new partner.
Before finding the shift in language, we need to see what the initial language is like. The first few lines have a lot of key words that might be used in a love poem. Just in the first five lines we see the words "smiling," "love," and "wedding."
However, there is also a darker undertone, for in these same lines we also have words like "ex-husband," "stuck," and "shame."
Spera goes on to describe the ex-husband, starting by explaining all the ways he could be the perfect husband. Even while she's describing, she foreshadows the later problems with lines like "But snapshots don't show what's beneath the skin!"
Eventually the tone begins to change. We start to understand that he was unfaithful and that many of the characteristics that would make him "the perfect catch" are also major flaws because they are related to his unfaithfulness.
For me, the main shift in language is when Spera uses the word "shit" in the line
Who'd lower herself to put up with shit
Though the entire poem, up to this point, has been talking about the things the ex-husband did wrong, this line is a turning point in that the speaker's anger and resentment has grown and reached a new level.
Her feelings haven't changed, she's just become more confident in expressing them. She somewhat returns to the previous language when discussing ways they could "work it out" but determines, ultimately, that she deserves better and is happier now that she has moved on and is no longer with the ex-husband.