This is a common philosophical saying people use to express how they wish they could have changed something(s) in their life. Perhaps they feel they had a bad decision, did something they shouldn't have done or didn't do something they should have. It could be as simple as turning right instead of left at some juncture in life. Who knows. It's not important the details per se.
What is important however is that while it is a good topic for discussion and self awareness, the reality is that in all likelihood things would be different from the start. All you think you would change probably wouldn't happen anyway. If you could go back and re-live the last 10-20-30 years knowing what you know today at the first point in time where you feel you made the wrong decision and now choose what you think would have been the right decision, then from that point on everything else will be different too! All the other bad or wrong moves and decisions you had subsequent to this will probably never happen.
By changing that one single decision your path from that point in time onward will probably have changed so completely that all the other bad/wrong decisions will probably never come up! Instead, life will deal you an entirely new set of problems and decisions you didn't have to face before. And, just as before, 10-20-30 years later you will look back and think about these decisions. Some you will certainly deem to also be bad or wrong ones too.
So while I think it's good to have some introspective on what you think you did wrong (and right), dwelling on how much better your life would have been had you made some other decision is pointless. Even if you could go back and redo that decision everything else will so be changed by that redo in ways you can't imagine.
Sooner or later you'll be back to saying "If I only knew then what I know now…"
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