I was interested by this topic so did a little research...
http://www.psychologytoday.com/blog/millennial-media/201301/living-loving-and-letting-go-in-2013
"My second regret then naturally follows the first. Too often this last year, I didn’t let go. There is that old song by the Byrds that goes “to everything there is a season.” Therefore I spent too much time wondering if I could go back in time and do things differently if things would have played out in another way. It’s the illustrious and endless, “what if” game that we play with ourselves.
But letting go is about more than the past. It’s letting go of fears, unrealistic expectations, doubts about ourselves and our worthiness in the world. It’s about freeing ourselves from the shackles of our minds. It is exceptionally hard work. In fact, I highly doubt this is an area I will gain any form of mastery over in the next few years or even decade. It is my greatest hope that I will, but in the meantime I’ve set this as my intention.
A close cousin to letting go is acceptance. Accepting things, people, situations for what they are and not forcing them to change allows for an analogous freedom. Throughout my training I’ve worked with a variety of people with an even bigger variety of personalities. Perfectionists, catastrophizers, and everything in between. I too, was a hardcore perfectionist once. In some ways I may still be. Seeing the anguish brought forth by needing things to be done a certain way I gave it up. It was simply taking too much out of me. Yet, often I work with individuals intensely preoccupied with crossing every “t” and dotting every “i.” Try as I might to explain these are needless worries to others, I cannot make them believe anything. All I can do is accept that we are at different places rather than try to change them."
IMO, we can not let go of something unless it is unresolved, however, that unresolved 'emotion' so to speak is usually within our own thought loops, your view on the present alters the past.