Linus:
Your description kind of makes me think of Hwayugi. One main couple and nothing really breaks them apart despite many people trying. There isn't an ex-girlfriend per se, but there are a couple trying to be current ones.
Maybe Kill Me, Heal Me would work? There is one couple and a woman from before who tries to wedge herself between the leads.
I've seen hwayugi and your right but towards the end they kind of do separate, and for kill me, heal me. I didn't like the fact that the ml had multiple personalities so I didn't check that out but thanks for the recs.