You've probably heard it 1000 times..."If i knew then what i know now..."
Lets examine that phrase and what it means for you psychologiclly
For the sake of arguement we will use the phrase in referense to women, but it doesnt matter the philosophie can be applied to whatever the phrase can be applied to.
So close your eyes and imagine if you will, this girl from your past. She was everything that you wanted and soo much more. Sparks are flying but for some reason things dont work out the way you'd like. You say something wrong, she moves away, she has a boyfriend...w/e. You know it was something special and your friends argee, and you find yourself thinking back and saying to yourself, "If only i knew then what i know now...".
You drift deeper into this day dream and imagine what itd be like had you known all this stuff. You woulda ended up kissing her, haveing sex with her, going out with her, etc. You imagine holding hands and exchanges of intimate moments...everything is rainbows and sunshine....then all the sudden like a tornado dropping from the sky you get pulled to reality by the simple fact that it wont ever happen that way, its been done and the resounding fact is that you didnt know then what you know now and you fucked it up. Now you just feel bad, negative.
Now instead lets look at this scenario...you look back at the same relationship, you take note of exactly where things went wrong and also note what went right. After analyzing the scenario you feel better knowing that you learned something from it and wont make the mistake again, whatever that may be. Much better right?
So here is the essence. The "If i knew..." philosophy always dead ends at feeling negative because you realize it cant be undone, whereas looking back at something, realizing the mistakes, and analyzeing things in an logical manner leads to growth.