He berates himself frequently, saying, "I should never have made that choice. What a bad decision. What a mess I made of things." While I completely understand that feeling and the whole issue of regret, it's not a good place in which to dwell. I encouraged him to feel the feeling and then let it go. We cannot change the past. All we can do is accept that what's done has been done—and let go of the regret.
What we can change is what we do going forward. We can also change how we view something. A different attitude toward the past doesn't change the past—but it changes us. It means more happiness going forward, and most likely it will help us make more thoughtful and better decisions in the future as well.
So if you're living with regrets, work on your attitude about the past—and mostly, work on the changes you want to make for decision-making and living your life in the future. Learn whatever life lessons you can from past decisions. What do you want to tweak and change? Create a plan for doing so. And stop dwelling on the past. Live today and plan for tomorrow.