That's Not Me

He was her first love, but she became the third party in his marriage. We obviously love each other but can't stay together. This kind of thing should have happened in the old society, but it happened to her who was about to graduate from college. The first love was ignorant, sweet, and bitter. For her, should she keep her own love or keep someone else's marriage? And who is the favorite in his heart, and who should he guard for his whole life?