I "gave up" a big fat lucrative cushy 15 year software engineering career. More like ran screaming from it.
I gave up my mortgage on a beautiful 1915 sun-filled charm-dripping house. As long as I'm in school, there's no chance I want to have that on my shoulders. It was too much house to deal with by myself even with a nice constant salary.
Which brings me to the giving up of the happily-ever-after story where I'd meet a guy and we'd be perfect together and have babies and support each others' careers. (Not to mention share the work of restoring the 1915 sun-filled house.) I went through a couple fiances in my 30s, and finally, really, gave up on having any kind of traditional family. Specifically, I gave up on prioritizing husband-finding, child-bearing and child-rearing over finding meaningful work, which hopefully will be medicine, given the choice of doing one or the other at age 40. Got my tubes tied because I'm not kidding around with this.
(Oh puhleeeze, dear SDNers, must you jump all over whether it's appropriate to surgically alter oneself and defy God's Special Grace in order to pursue a younger person's career, especially if you feel compelled to convince me that it's not too late at all to still have babies? I'm 40, for chrissakes, and I got the OB to take some really cool pictures of my ovaries while he was in there.)
If I have any regrets, I wish I would have believed in myself earlier, bailed out of software earlier, not tried so hard to get married & pregnant, been quite a bit smarter with money, and studied harder whenever a GPA was on the other side. I also regret not taking advantage, as a child, of my mother's complete willingness to teach me Spanish and piano. She's fluent at both; I largely suck.
It's always going to be weird, if not specifically a regret, to be the old chick in the program. I expect to be too busy, and quite possibly too happy, to regret much.
Hope this helps somebody. Best of luck to y'all.