General Comment:
You wont know how good it was until to late, when its gone - very true and something most people felt im sure. But, gone here could mean break up or even death, the song never reveals... still the lyric is cryptic about who is in fact letting this go, does he break up from dissapoinment or did she let him go and in lack of action he just lets her go instead of fighting for it.. But it could be also that loss puts you inthis state the lyric describes and the message 'let her go' simply means that you have to try to carry on because the pain lingers on