I don't do regrets. We can only take decisions based on the info we have at the time. If we didn't have the info, it was out of our control. No reason for regret.
If we did have the info, we still took a decision based on what we thought was best back then.
I love your suggestion to focus our energy on the future instead.
Mindtravel to the past is a waste of energy :) (and it's how all the MLM companies sell their coins: Imagine how rich you could be today! :D )