I think I would tell myself that I really didn't need 3 majors in college. I should have dropped the English one down to a minor since I went to grad school. I don't know what I would have done in the meantime with all that extra time, but I guess I could have taught for a year before I moved to NYC with DH. I don't think I should have moved in with DH sooner since his life was tumultuous, so I probably would have kept that timeline or even moved in a bit later with him. It might have set me up to be a better place financially. Other than that, I can't say I regret my time in NYC and I learned a lot and got a graduate degree for probably the cheapest one can in the entire country.
I also probably should have sought treatment for my PMDD sooner.
Don't marry husband #1 and get your MFA in creative writing instead. Or apply to the music conservatory your college voice teacher recommended.
I have a hard time with this one, because if I'd done that, I wouldn't have met husband #2, who is practically perfect, and had my kids, yada yada. I mean, my life turned out all right. But I will always wonder what would have happened if I'd been more ambitious and nurtured my talents.
Divorce your exh sooner! Don't be miserable for years and just wait for him to blow it all up with an affair. Get out while you're still in control. (Or really, don't marry him at all if I'm being honest. But then I wouldn't have my kids)
Stop being in such a rush to have a settled, grown-up life. Try that college class that doesn't fulfill any hard science requirements. Read nonfiction. You don't have to go straight to grad school. 20 is not too old to learn computer programming.