because it is only when we look back that we understand there were other choices we could have made that would have significantly changed our outcomes.

When we look into past we have feelings about we didn’t do but could have done. However when we were actually facing the situation we made the choice that seemed right in the given circumstance and emotional status. In the current situation we often fail to see many things that we see later and then regret our past decision. There is no benefit of regretting. Taking the vision from the past if learn to see the future better that's a gain as well.