I’ve recently finished binging a Chinese or Taiwanese drama called “What is Love” in which the playboy ML is obsessively and truly only in love with the FL.  FL is an ordinary and poor woman, but strong, optimistic and always cheerful in life. The ML, although he’s slept with many women, but his heart truly only belongs to the FL and is trying hard to win her heart, but she’s not believing him at all, and doesn’t believe that men like him could fall in love with one woman. The SML is in their way a lot, being kind to her, showering her with gifts and attention and both are very rich, powerful and handsome men. I’d like  some rom-com recommendations like this one please.

The first thing that comes to mind is Love Rain. I think this is almost exactly what you're looking for. However, there's also a lot of melodrama related to their parents, so only 5-10 episodes (I don't remember exactly) are really about the good girl-playboy dynamic.

Some other dramas with playboy MLs, but where the conflict isn't necessarily related to whether he will be faithful.

A Love To Kill

My Secret Romance - Of these 4, this is probably the closest to what you want.

Last Cinderella

The Innocent Man - Not exactly a playboy, more of a gigolo.

The only one I can think of is: Sixteen Syndrome

Chris Wu is probabaly the only reason I watched What is Love. 

And i second Que Sera Sera, also maybe check out:

  • Plus Nine Boys
  • PS man
  • Hateful But Once again