We have been taught that time only flows forward. There is, however, evidence that it flows both ways, forward and backward. As we plan for the future, we also reflect on the past, memories both good and bad, thoughts of what if, why and how.
Planning and remembering are powerful tools, best used for guiding outcomes. Regret over the past is not as helpful as a reflection of the past, to better plan the future. Remain optimistic and kind to others, but most importantly, be kind to yourself, forgiving others weaknesses as well as your own. This goes a long way towards a happy, healthy future.
The future often alters how you perceive effects of the past. Today, if you feel fulfilled and peacefully happy, you may reflect on the past and say, "both the positive and the negative of the past helped to form who I am now, and it’s good." If it is true that perception forms reality, then the past is formed by our own current perspective.