I watched this k-drama a while ago, pretty sure it's pre-2010 era. All I know is main male and female leads constantly fight with each other, the guy must go to army at some point. And I'm pretty sure the opening was this song by Yui : 

Sorry, that's all I remember , plz halp

I'd recommend you to check out this page: https://www.yui-lover.com/biography/
it has all the dramas/movies/animes listened that used her songs. Maybe it is underneath that pile ;)

Thank you Lou. I'll check it out