Multiple ways to let go of regret I know of that have been useful at different stages of my life. I have no idea how they would play out for you:

• Ask yourself was there anyway to predict the future that happened at the time? If not (do you have a crystal ball?), then why regret?
• Label that nagging voice inside of you with a personality. Call him “Perfect Pete” or something, he always thinks you should get everything right, be super brave, well, is that realistic? Giving it a silly name helps.
• Accept you don’t write life, life writes you.