“Only know you love her when you let her go
And you let her go”

Passenger – Let Her Go Lyrics on LyricsMode
EXPLANATION
You will only really understand how much you love someone until you have lost someone. and he let her go, so now he understands how much she means to him.
EXPLANATION
You only know you love someone when you let them go. Just like things, you will only know their values when they're gone.