she made lots of promises , but none of them were as strong as "I promise I won't love anyone else but you."
as she promised, he did too, as time flew by, her innocent heart stayed with him, not knowing surprise! it's time to let go of those promises, because, boom! he found a new one to love and to keep in his heart.
Which means by it's time to say goodbye To all the words and promises you've kept in your heart.