Some of my regrets. - not brushing my teeth when I was a kid (not really my fault though). I'm paying the price now - I was very good at maths when I was an undergraduate. I had to choose between applied maths and CS. I chose CS but I think it gave me lesser career opportunities. And it's easier to learn CS on the side as a hobby (it's constantly evolving so you have to do it anyway). - I didn't pick the right PhD topic. It seemed cool at the time, but it was a niche field with little practical application. - I should have done my MS abroad (but I was afraid that my English wasn't good enough, among other things).

Overall, I didn't really know what was possible at the time. I was only looking at step "N+1" and missing the big picture.