You can’t have a regretful thought and a grateful thought at the same time, so why not spend the time positively? Every morning and every evening, make a list of what you are grateful for. Get a gratitude buddy and exchange lists so you have a witness to the good things that are happening around you.
Overthinking is something that can happen to anyone. But if you have a great system for dealing with it you can at least ward off some of the anxious, negative, stressful thinking and turn it into something productive, useful, and effective.