break up (with someone) /breɪk ʌp/ (phr v.) to separate into smaller pieces Example: They broke up after 5 years in a relationship.
first love /fəːst lʌv/ (n.) Example: He's my first love.
flirt /fləːt/ (v.) to behave towards somebody as if you find them sexually attractive, without seriously wanting to have a relationship with them Example: She's trying to flirt him.
love triangle /lʌv ˈtrʌɪaŋɡ(ə)l/ (n.) a situation that involves three people, each of whom loves at least one of the others, for example a married woman, her husband, and another man that she loves Example: He cheated her. He was in a love triangle.
lovelorn /ˈlʌvlɔːn/ (adj.) unhappy because the person you love does not love you Example: There are two poor lovelorn souls here who don't fit in.
lovesick /ˈlʌvsɪk/ (adj.) unable to think clearly or behave in a sensible way because you are in love with somebody, especially somebody who is not in love with you Example: I wanted to sigh like some lovesick schoolgirl.
lovey-dovey /ˈlʌvɪˌdʌvi/ (adj.) expressing romantic love in a way that is slightly silly Example: A young couple was in there acting lovey-dovey.
unrequited love /ʌnrɪˈkwʌɪtɪd lʌv/ (n.) is love that is not openly reciprocated or understood as such by the beloved Example: Only so many poems can be written about unrequited love.