Leads are dating. She breaks his heart because she falls in love with someone else and leaves him.

Any where the 2 leads are together. The female lead then breaks the male lead's heart because she falls in love with someone else. She gets together with that person. Eventually she realizes she still loves the main guy and tries to get back together with him. In the end they do. And I don't want ones where she has trouble deciding. She falls in love with one and then falls in love with the other. 

Scarlet Heart Ryeo

Bu Bu Jing Qing

I miss you

My love Eun Dong - (fell in love as kids,  got separated but they met again as young adults only to be separated again, they meet again she doesn't remember him, she has a husband and child)

Coffee Prince - The 2nd leads.

I need romance 2

* My Sunshine - Though it's a bit different of what you're asking, FL & ML was in a relationship when they were teenagers but due to misunderstanding, FL left the ML and went abroad. FL got married then after 7 years go back in her country & met the ML again. ML pursue her with vengeance but still loving her at the same time. Similar to what you're looking for.

 MichinYeoja:

Our Gab Soon 

I personally don't like when one of the leads love another though.. It feels so fake that I can't keep watching:(

Eh, I do. It makes it all the more better when she finally falls for the other lead.