What would you have done differently if you had to go back?
I'd have had more sex ... though it's hard to see how I would have figured that out without becoming a total jerk, without messing up so much like I actually did.
This is pretty much the problem with this sort of thing - some stuff you just have to work out the hard way.
The trick is to avoid doing stuff you'll feel upset about people finding out about - thus: no regrets.
My undergrad degree was Engineering - no Computer science, no physics ... perhaps what I could have done at the start was to settle on something sooner: looks better for scholarships and on the academic record - but then I wouldn't have such a broad experience base which lead me to more varied and interesting tasks, and I would not have developed the outlook I have.
Speciality was Solid State - but really it was "weird problems". Stuff people didn't see how to handle right away like modelling bacteria growth in food, figuring out how to reduce the foam in beer vats. Things you wouldn't think of as physics problems. There was one about designing bikini strings to handle big boobs better (this one had a grant) but someone else got it first. Poor guy, having to work in close proximity with scantily clad models ... anyway, snap out of it, it also lead into stuff like law and education.
What I learned about solid state is that it is basically boring - I would advise me to push harder for more interesting problems.
Mostly I think I'd advise me to take more advantage of my supervisor - really be a pain in the butt.
But if I'd done that, I'd still be an academic today ... probably would not, i.e. have toured the country with Richard Stallman.
I would probably be much more of a hard-ass too ... not so laid back. And I wouldn't be here.
The lesson I want to push is:
The mistakes you made in the past made you what you are, it is the same for the future mistake ... if you are at the other end of the journey I've just taken, then do not be afraid to make mistakes. Don't be afraid of other people finding out about your mistakes. Don't be afraid to be open and honest - but be smart about it. Follow your passion - intelligently. Do not regret - it's all AP. Apply what you learn - everywhere.
Oh... and fall in love lots.