For me I have several-things I regret from my past like not making Youtube-Videos sooner as I have had a Youtube-Account since 2008 but I did not actually start making videos until nearly a decade later as I could have become successful and financially-autonomous by doing just this.
I also regret not going to more rock-concerts in the past as well because of all the oldschool-rock bands and musicians now threatening to retire from the road for good like Kiss, Bob Seger, Lynard-Skynard, Slayer among others and I still am having-anxiety now and than I might not get to see them perform live before they actually call it quits due to geographic-distance combined with lack of financial-means.
I also regret that I didn't make a novel or story after I made one for my High-School senior-project in 2006 as that could could have been another source of success and income for me.
How about for you what are things you regret most from your past and why?