Really puzzling question, considering the fact that life has no ctrl+Z button that one could simply push to write some wrongs, get some favours and lots more.
But to be frank with you, as strange as it sounds I've got to say this if you don't know by now that I'm quite an ambitious man and I give 100% to whatever I do and find rewarding provided it's legal.
A few months ago I got engaged in a gambling business and I was quite good at it from the onset and so I made a few huge successes here and there but also major downfalls, which is one I'll never forget.

I vividly remember that faithful day when I made a certain stake on a football match prediction with a large payout if the games played out to my selection, that is if I did hit jackpot. The most saddening part that scares me the more than anything is moment was the very day I experienced the most disappointing experiences ever! I needed just one more selection to go in favour of my predictions , but unfortunately I got hit with a stroke of bad luck which denied me a whooping $2,500,000.
So if you should ask me right now I'd say that if I could change just one thing in my life today, definitely it would be the opposite selection of that gamble on that faithful day.
It may sound strange to you, but that's a fortune to me.