What is your deepest regret?

I did follow my dreams, and none of them worked out the way I hoped.

I wish I'd chosen the first offer I had from uni (just after matriculating) - to study English Lit and Psychology as majors, with a few other humanities as minors. I would have gone into psychology as a career and most likely would have found it profoundly satisfying (despite the many challenges).
Writing and psychology were recommended from my vocational tests - I would have been wiser to recognise the value of that.

Truth is, when we're young we don't yet know enough about ourselves or life in general - so it's natural and normal to make many mistakes.
Many of us learn our lessons the hard way and far too late.