break up (with someone)
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
first love /fəːst lʌv/
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
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
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.