As soon as any event ends, it's gone forever. No amount of thinking will prevent it from having happened, eradicate it, or change it in any way - yet we waste a lot of time & energy trying to accomplish these impossible tasks.
No amount of planning guarantees perfection. Most of our time obsessing about the future is not productive planning - instead we catastrophize - anxiously hypnotizing ourselves to have a negative outcome.
The best, healthy, productive use of our time and effort is being fully present, fully engaged in our present activity. The only way to do this is to let go of the past, let go of the future, and embrace the now. Though such "presence" is clearly the very basis of mental health, it's a rare phenomenon. But presence is a trainable mental habit - an important part of mindfulness training.