Leaving medium corp for big corp. I didn't give medium corp a real chance and I felt like I was on an island by my own. I should have reached out more. When big corp came along with more money I left. It was a mistake and I really regretted it for a while. Luckily, I landed somewhere I like (for less money).

In general, I wish I would have started earlier. I was afraid to learn to program because I felt I was bad at math. I think my teen self would have loved programming just as must I do today. It probably would have helped me do better in school. A lot of math concepts I didn't understand became easier as I studied programming.

I don't really regret any technology choices I have made developing something but I now regret investing so much time into a few things that never really took off (a game or two and an app or two). At the time I told myself that I was doing it for my own sake (I was) and that any traction would be a bonus. In retrospect, I should have put more time into understanding if I was really building something other people wanted.

You know what though? Setbacks are irrelevant. Unless you are actively trying to screw things up you probably acted on the information and best intentions available at the time. No sense beating yourself up over past mistakes.