In general, I try not to look back with regret on most things in life. One way or another, those experiences helped me become who I am today.
Having said that, I wish that I knew how to truly maximize value for cc currencies sooner. I was always good at racking up huge amounts of reimburseable business meal expenses on my citi forward card, but I look back now and cringe at the hundreds of thousands of TYPs that I redeemed for only 1 cpp via "mortgage payment." While 1 cpp for cold, hard, cash seemed better than many other less attractive redemption options from Citi, I also didn't realize the full value in redeeming for travel back then.
Granted, there is always a balancing act when one is cash poor and points rich, but my outlook on many things has changed as the value of experiences have grown to far outweigh material objects.