Male and female lead begin to fall in love, but then her ex reappears...

Kinda like Witch's Romance and (to a lesser extent, more the ex reappearing) The Time We Were Not in Love. 

The male and female lead become close and as they fall for each other, the ex comes back. That makes her conflicted. It causes problems between her and the male lead. In the end, it works out and the male and female leads finally get together.

Gi Ho's Starry Night - https://mydramalist.com/16921-go-hos-starry-night

My Love Sam Soon - https://mydramalist.com/221-my-lovely-sam-soon

I am not entirely sure, but think this one also fits the bill. 

Personal Taste - https://mydramalist.com/19-personal-taste