I listened to Joowon's cover in 'A Song For You'. The song was really sad and the melody was really good. I searched for it's lyrics and in the end, I found it. Credits to Soompi.

Shin Seung Hoon (신승훈) - Butterfly Effect (나비효과) [TRAN]

If I could predict tomorrow's events
Would I be able to rid myself of regret?
If only I knew what I know now
Before I let you go

I grabbed the remote to watch TV
But I turned it off because the drama was too sad
Staring into the empty screen,
I realized that my love had turned off also

Because I wanted to be alive
Because I wanted to be with you
I left a lot of me behind

*I don't have myself anymore
You have all of me
I want to let out my anger, but you're not here

It was a foolish love
But love is foolish
No matter how much you offer, you feel guilty
Every day is filled with a void
In the end, you end up alone

What I know now is what I learned after you left
If only I had known all of this before you left...
If I had, maybe we would have been having tea right now*

Because I wanted to love you
Because I really wanted to be with you
I left a lot of me behind

* Repeat