I'm pretty sure this was an adaptation from a JDrama (or is it TwDrama?). It should be good for it was adapted into a KDrama.
Title: 프로포즈 대작전 / Peu-ro-po-jeu Dae-jak-jeon
Genre: Romance, fantasy
Broadcast network: TV Chosun
Broadcast period: 2012-Feb-08 to 2012-Mar-29
Air time: Wednesday & Thursday 20:50
Original Soundtrack: Operation Proposal OST
Related TV shows: Proposal Daisakusen
Kang Baek Ho realizes that he is in love with his best friend, Ham Yi Seul, on the day of her wedding to another man. He travels back in time to various events of their lives hoping to change the outcome of their relationship. Will Baek Ho be able to change the course of their future?
Yoo Seung Ho as Kang Baek Ho
Park Eun Bin as Ham Yi Seul
No Jung Ui as young Ham Yi Seul
Lee Hyun Jin as Kwon Jin Won
Go Kyung Pyo as Song Chan Wook