It's a yaoi manga and the story goes like this:
The MC meets a broken-hearted high school boy because of an unrequited love. He then remembers he had the same experience back then.
Turns out that the teacher of that boy is the MC's ex-crush/ex-boyfriend (i forgot). If i'm not mistaken the ex is also about to get married but it's cancelled. In the end they both end up together again and the high schooler also end up with his crush.

Ikujinashi no Koi 🙂