You know these moments when you make a decision and then after some time you look back to judge the decision you make and you say "Man. I was sooo dumb"? You shouldn't really feel bad about it
You assume that you had the same information as you do right now, but that is not true at all. These regrets and bad feelings are just of no use. Instead, you should really ask yourself "What can I do from here?"
The message is just this quote here:
"Finish each day, and be done with it. You've done what you could; some blunders and absurdities no doubt crept in. Forget them as soon as you can. Tomorrow is a new day. Begin it well and serenely, and with too high a spirit to be encumbered with your old nonsense"