The main reason is that there’s nothing you can do with that regret. Regret is one of those things that by definition you would have to be thinking about the past to feel. When I was in AA I basically learned how to only focus on what I can currently make a difference in. If it’s tomorrow, I won’t be able to deal with it until tomorrow, let me have todays sanity. If it was yesterday, there’s nothing I can do to change what I did, and I can’t fix my mistakes if I’m in a bad place mentally because of regret. Let me have todays sanity.