The truth about the past is simply that it has passed. This means that our experience of each moment -- for its pleasure or pain, peace or trouble -- is a direct reflection of what we are in relationship with in the present.
Our attention connects us to life; it establishes our relationships with all that unfolds around and within us. Our experience of life reflects these relationships just as a rainbow reveals the different colors of light. This means that, whenever resentment or regret darkens our day, there's only one reason for our sorrow: we've been tricked into revisiting and reforming an unconscious relationship with some misery-making moment from our past. We've simply made a bad connection within ourselves, due to a temporary lapse of attention.
If it helps, think of troubling times like these as your having been distracted while taking a long walk through the city. Maybe you were caught up with thinking about the "good old days!" Then you suddenly realize that, in your dream state, you've wandered by mistake into a dangerous neighborhood where you know that bad things happen all the time. What do you do? Do you stand there and hate yourself for having sleepwalked into a nightmare? Of course not! Awake now to the danger you've put yourself in -- and just get out of there!
We can call on this same simple action whenever our attention wanders and takes us into the bad neighborhood of our painful past. In other words, if the resentful or regret-laden state we're feeling is due to a bad connection, we just deliberately disconnect ourselves from that dark thought by realizing where we are and how we got there. By reclaiming our attention -- by bringing ourselves back into the present moment -- we are released from our unconscious relationship with the past, putting its pain behind us, where it belongs. There's nothing more to it than that -- other than to enjoy how good it feels to be free.