The speaker is caught in a painful contradiction: they tell someone they don't want them around and don't care, when the truth is the opposite. The first two stanzas set up this mismatch with simple pairs of lines, "I say" against "I really, really do." The repetition makes the gap between words and feelings easy to see.
By the second stanza, the pretending has started to backfire. The speaker admits they now believe their own lies and no longer know what is true. They also sense that thoughts of this person will "define my youth," which suggests a first, intense attachment that will stay with them.
The last three lines are shorter and more direct. The ache arrives when the person is absent, and the heart "begins to break" at the moment of leaving. The mood is wistful, vulnerable and a little regretful, like someone who realizes too late that hiding their feelings has only made the hurt worse.