Someone has walked out of the speaker's life, and the poem is what's left behind. In short, flat lines, the speaker lays out how it went: a person arrived, said sweet things, and made the speaker feel claimed. Then, with a shrug of a line, "I guess you got bored," it ended. The speaker doesn't seem to have been given a reason, only left to guess.
The feelings are mixed. The title, "Liar," in quotation marks, suggests the speaker now sees those sweet words as false, yet the line "I miss you," still shows real longing. The comma after it, with nothing following, feels like a pause where the speaker doesn't know how to go on. Then comes the question, "Do you miss me?", answered right away with "I highly doubt it." That bitter reply hides hurt behind sarcasm. The mood is quiet, raw and resigned, like someone trying to sound unbothered while clearly still stung.