'If only that hadn't happened...' 'If only this had happened instead...' 'Why, did I ever agree to X?' 'What was I thinking when I decided to do Y' Okay. Those examples will do. Such thoughts, no matter how sincerely uttered or how wistfully felt, are not just counter-productive, they are potentially destructive. They stop us from letting go and moving on. They cause us to dwell in a realm of might-have-been and should-have-been. And you've got too much positive potential in front of you now to waste time with any of that.
You have so much to be proud of, to be pleased with and to be grateful for. And why do you think I am reminding you of this,  how cynical! Do you really think that I would only remind you to count your blessings at a time when you might feel inclined to question whether you have had any blessings to count? Well,  It's true that currently you can more easily see the wrongs than the rights and you are more aware of problems than of opportunities. But that's nothing that a small change of perspective can't fix.