I would say my biggest regret was rushing to university immediately after high school (even though I graduated uni and with a solid gpa in a STEM field - Computer Science) because everyone else was doing it at that time and also due to parental pressure. I feel like if I could go back, I'd at least get some job experience and work a bit then decide if I really want to go to college to get a degree and then get that high paying job afterwards. I'm saying this because back then, as a naive dumb teenager and young adult, I foolishly believed that all I needed was good grades and good degree in a promising field then I'd get a job, but clearly this isn't 1960's, 70's, nor 80's anymore. It was 2012.
I'm saying this because today, I have lots of student loan debt (over 35K with interest) and I'm on IBR, which means the more I make (that's even assuming that I do get a lucrative job/career), the more I pay back. As of now, I have a small part time job that pays enough to put food on the table and have a bit of spending money for some entertainment and that's about it. I also ration out what I eat and micro-manage my spendings to make the most out of my limited finances. Had I accurately seen what the world is today, I would have told me 2008 self, delay (or don't go) college until I know what I want and am more financially and professionally prepared (with job experience already under my belt).