You said that you could let it go
And I wouldn't catch you hung up on somebody that you used to know
Now we see a different usage of “somebody that I used to know”. It’s possible it isn’t just a phrase for just this particular girl, it’s possible he uses it for all the girls he broke up with. When he was with his most recent girl, he kept assuring her that his former girlfriend was just someone he used to know; until she caught him going back to her.
The other possibility is that the somebody he used to know is still referring to her. She hears him going on about how she wronged him so badly and he’s so done with her, and she heard him say he didn’t care about the breakup. But now he’s clearly emotionally distraught and shows he still has feeling for her.
Either meaning indicates that he isn’t done with her forever like he claims, and also that it wasn’t like this girl was the only one at fault during all this.
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