Yay, hypothetical time!
(everyone who hates my posts, please ignore this, because its not going to be any less rambly and speculative than usual) Long story short: I agree with Mr. BAW.
Memory is a strange thing... I think the question assumes that people remember every single decision they've ever made, and that they remember the rationale and circumstances of that decision in absolute 100% accuracy. I think that it's an impossible undertaking. Think about how many decisions we make, both conciously and unconciously, every single day... and then apply that to every day of your life.
It's also a pointless question insofar as if you changed every decision you ever made, you'd be an entirely different person. Even if you changed only the ones you thought were 'bad decisions', you have no way of knowing that those decisions weren't the best possible decisions you could have made at the time -- there's no way to know that, if you had changed something whether your life would likely be better or likely be worse.
Passivity, as far as I've been able to learn from my experience, is not as significant an agent of change as activity - if you want something to be different, make it so by being an agent for yourself. If you look back on the past and decide you don't like who you are, you're the only one who can change you. It'd be a nice, easy, quick fix if you could just wish something had been different and have it be so but it's really not realistic. If we could just change the past, why would we have the capacity to learn lessons from experience anyway? Experience would be irrelevant to life, we could just go back and change things to get what we want. It'd be like snapping your fingers and having what you want without having to work for it. That seems like nothing but fantasy to me.
Mistakes will always be made, its a necessary fact of life, and it shouldn't stop you from living your life. I try to just keep on and to try not to let the minutiae of everyday obscure my sense of where I am and where I want to be going. The past is gone and it's not coming back so I don't want to worry about it.