BSS

BSS stands for ぼくがさきにすきだったのに / Boku ga Saki ni Suki datta no ni loosely translates to "Even though I loved her first" which is a subgenre of NTR where the main guy and girl aren't officially in relationships such as girlfriend or wife, but the story still evokes feelings of jealously and loss when the main girl ends up with someone else"

Technically, this scenario doesn't even qualify as NTR. The term you're looking for is BSS.