"if you let it grieve you, your youth will surely leave you and,_ _ _ _ _ _ _ _ _"...?

Answer 1

these should be the best years of your life

Answer 2

This is the song "Best Years of Your Life" by Carl Smith.

Lyrics: (And these will be the best years of your life)
You say your heart is broken that my love was just a token
You say this parting touch you like a knife
But if you let it grieve you your youth would surely leave you
Cause these should be the best years of your life
I know it won't be easy cause your memories are busy
And yet you're still too young for all the strife
So let me tell you like a brother go on out and find another
And these will be the best years of your life

You say that taste is bitter because you loved a quitter
But I'm not free to take you for a while
You're lucky that it's over cause somehow you recover
And these will be the best years of your life
I know it won't be easy...