20/20 Hindsight

I have to work on living my life without constantly looking over me shoulder. The problem is, I'm my own worst enemy when it comes to the phrase "I should have done." I'm my own worst critic, and it doesn't matter what I do, I can always think up some reason for why something else would have been better.
Sometimes I worry that I spend too much time looking backwards and not enough looking forwards. But this, too, leads to the ever-constant cycle of "Should have done instead". Sometimes it's like a roller coaster that I just can't seem to get off of, no matter how hard I try. It's a constant cycle, a continuous nightmare.
I'm slowly learning how to slow down and stop my tendancy to constantly second-guess myself, I have learned not to change my answers unless I am completely sure that I know the right one this time around, but still, it's hard. Some habits never die. I hope this isn't one of them.
I want to look forward. I'm tired of looking back. It's over, done, I can't change it.
So then why must I constantly think up "better" answers?
Why can't I just live?