Basically, FMC found out she had a terminal illness so she broke up with MC without explaining why and just said "I wish I never fell in love with you" Then she miraculously recovers but can't reach MC since he understandably blocked her everywhere.

Meanwhile MC has made some bad career choices that lead him to overworking a lot without much gain and after the only solace in his life, his gf, breaks up with him suddenly, he sorta gives up and actually overworks until he dies and goes back 10 years in time.

Which brings us to the current timeline where the MC says to himself that he doesn't want anything to do with her since he's still shaken up from the breakup and FMC keeps pestering and stalking him to further their relationship and block anyone else from coming close to him. Frustrating part is that MC allows her to dominate his life instead of cutting her off, even when he knows things wouldn't be any different even if they start dating earlier this time. And there should be no way of convincing him on why FMC broke up with her since he wasn't given a reason in their last life. Even if she reveals she's a regressor like him, since there is no way to prove, for all he knows she was just bored of him the last time and now just lying to get what she wants.

So maybe this is the reason why there is usually a single regressor. Since it makes the other party look like a chump, especially when they can't see the ex's inner thoughts like we can.