Regret derives from failing to do what you wanted to do; not acting in a manner you desired or failing to say what you meant to say when an opportunity presented itself
Regardless of whether the action was right or wrong, regret is the reflection of that which we have perceived as a missed moment in time that has now culminated in an emotional sense of loss
In my opinion, it is a total waste of emotion as the reality of “what is” must be the focus. Reflection for the purposes of growing and learning is a more valuable use of one’s time and energy.
Therefore, waste few precious minutes in “what was” and seek to grow yourself in the here and now – “what is”.
Adiuva me verterem figuram mundi 
lifemeaning4u - Mavrik 