The speaker is scolding herself, or himself, after a breakup. The poem is built from a short list of hopeful beliefs: you were special, he still loved you, he would come back. Each one starts with "you thought," so the repetition feels like a person counting up their own mistakes. The line "you thought so much" is the turning point. It admits that all this hoping was really a pile of wishful thinking, and it hints at how much time was spent on it.
Then comes the blunt ending: "but you're wrong." It's set apart from the rest and delivers no comfort at all. The speaker uses "you" instead of "I," which suggests a kind of distance, as if they have to talk to themselves from the outside to accept the truth. The opening "mhmmm" sounds like a sigh or a weary hum, which sets a tired, resigned mood. The overall feeling is quiet heartbreak, with a bit of self-mockery mixed in.