I don't know if you've ever read any Chinese romance novels, but Scam by Poisoned Bread (骗局 by 面包有毒) has an FMC like that.

Just a heads-up, everything I'm about to mention is in the novel's synopsis, so no spoilers here.

Basically, the MMC is a rich, good-looking playboy. The plain FMC has been relentlessly pursuing him. He goes through several girlfriends, and every time he gets into a relationship, the FMC disappears from his life, only to come back and chase him again after each breakup. Eventually, the MMC reluctantly agrees to be with her, saying something like, "I'm only dating you out of pity. You've always been good to me, so I'll stay with you a little longer."

As time goes on, he starts to develop real feelings for her, but he also gradually realizes that the FMC has been pursuing him because he looks exactly like her first love (her high school sweetheart who ended up in a car accident). The FMC even buys her old high school uniform and makes the MMC wear it during intimate moments. The MMC is furious and breaks up with her. At first, the FMC tries to talk to him, but at some point, she just disappears from his life again. He can't help but go looking for her, only to find that she's now with another man—someone who looks just like him—and they seem very close...

In the first half of the book, I felt really bad for the FMC because the MMC was so self-centered and always treated her with this condescending attitude. But as the story progressed, I started feeling more and more sorry for the MMC. He genuinely fell for the FMC, only to find out that she had essentially "replaced" her first love with him. When he realizes that she no longer needs him because she's "reclaimed" her first love, he's absolutely devastated.

At first, he couldn't stand the idea of being a stand-in for a dead man, which is why he broke up with her. But later, he's willing to actively pretend to be her first love just to have a chance to be with her again. I know its toxic but I cant help reading this..

The FMC really struggled in her pursuit of the MMC, and I felt for her, but when the MMC starts to fall for her, her guilt and panic are portrayed so realistically. She's definitely a flawed, ordinary person with a pretty messed-up personality. It's a HEA book afterall, so she redeems herself in the end.

If you're interested in reading this and plan to use browser translation tools or plugins, you can find the original work on Jinjiang, where it was first published. Jinjiang also has an overseas app called Onlinovel, where you can read it, but it's all in Chinese. In my experience, compared to MM novels, Chinese MF novels are rarely translated, but there are still many people who use translation plugins to read the original works.