No resentment or regret exists without your being tricked into revisiting and reliving some painful mental picture from your past.
Think about it. Does the echo of some spoken word have the power to change the voice that speaks it? Of course not. That which was "once upon a time" -- regardless of its nature -- has no authority over us in the present moment. 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. A brief example shows us the truth of this simple, but critical, idea.
Two friends go out to share a picnic basket on a warm, beautiful spring day. They spread their tablecloth over a soft rolling hill, taking their meal in the middle of an ocean of green whose waves are colorful flowers. As they dine, one of them looks out across the open meadow, drinking in the unspeakable delight of so much natural beauty. Giving her full attention to the rainbow of life revealing itself all around her, she thinks, "No one is more fortunate than I," and she wishes the whole world could know the peace and happiness flowering in her heart.
At the same moment, sitting less than three feet away from her -- surrounded in equal measure by this living bouquet of spring flowers -- her friend has a completely different experience! His attention is riveted on a horrible experience he had the day before. He can't seem to shake the image. In his mind's eye, he sees the face of his department superior, smirking with glee, telling him that he won't be getting the promotion he expected. And worse, it went instead to the new "hotshot" he can't stand, let alone that he'll now be working under this kid!
Here's the point. Although the man looks at the same open expanse of bright flowers as his friend, they see nothing in common. While she wishes everyone could know her joy, he wants only to escape his pain. He is the unwitting captive of nothing but a self-induced dark dream. Improbable though it may seem, the bitterness he feels in reliving this moment is inseparable from his revisiting its emotion-packed memory. Life isn't punishing him; he is punishing himself by looking back at what he then wishes he didn't have to see!
Perhaps the day will come for this man when he grows weary enough of this kind of self-created conflict to realize the truth of his condition: the weeds of resentment and regret cannot grow in the light of any mind that knows what -- and what not -- to give its attention to.
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.