There comes a point and time in everyone's life when they need to let go. It's easier said than done, and even when most recognize the need to let go they can't do it.
Letting go is about freedom; freedom that can only be experienced through the disconnection of the things - or the person - that cause the mental imprisonment. It's about clearing a path that will take you from where you've been, to where you want to go. It's about stepping out of the shadows of not-so-good-experiences and emerging as a victor. It's about having more of you to offer, and offering more of yourself to love.
Yesterday does hold lessons. Before you can move on from anything or anyone, you must first truly embrace the reality of where you are and how you got there. Was it due to poor judgment? Bad decisions? Impulsiveness? Personal or professional growth? Shift in values? Was it an unhealthy or unproductive relationship? A non-reciprocal friendship? Whatever the case, it's imperative that you know where you are so that you can avoid a return trip to this place of adversity in the future.
When a habit or association imprisons you to an undesirable fate, or clouds the brightness of your present and future, it's time to let go. Your motivation to do so is directly correlated with your desire to be free; free from the adverse conditions that your choices have, or will create. Letting go is a challenge, but it's always a choice.