In "I guess," the speaker looks back on a relationship with someone whose smile was so captivating it pulled her in. The phrase "I guess I did" sounds almost like a shrug, as if she's still working out how it happened. What follows is the sting: he used that charm against her. He didn't just enjoy being liked; he took advantage of it.
The speaker then turns the focus onto herself. Her heart was the kind that "believed all the lies," which suggests a trusting, open nature that he could exploit. The repeated "I guess" at the start and end frames the poem like a quiet bookend. The first one admits she was taken in, and the last one admits he understood exactly what he was doing.
The mood is tired, rueful and a little bitter. There's no shouting or dramatic accusation, just short, plain lines. That restraint makes the hurt feel real, like someone saying something painful in a calm voice because they've had time to think it over.