The speaker is nursing a broken heart and has come up with a painful plan. Loving someone, they say, was something they had to learn, and now they believe they must learn the opposite: how to hate that same person. The word "learn" in the title and in both lines makes love and hate sound like skills, things you study and practice rather than feelings that simply happen.
The second line explains why. The speaker's heart "doesn't un-learn," so it can't just let the love fade. Forgetting isn't an option, and hatred seems like the only exit left. That's what makes the poem sad rather than angry. The speaker doesn't really want to hate; they're reaching for it because they feel stuck.
The mood is quiet, tired, and resigned. The plain, almost matter-of-fact tone suggests someone who has run out of better ideas, and the "'Cause" in the second line gives it a spoken, confessional feel, as if the speaker is admitting this half to themselves.